Titanium dioxide thin films for high temperature gas sensors

Titanium dioxide (TiO₂) thin film gas sensors were fabricated via the sol-gel method from a starting solution of titanium isopropoxide dissolved in methoxyethanol. Spin coating was used to deposit the sol on electroded aluminum oxide (Al₂O₃) substrates forming a film 1 μm thick. The influence of crystallization temperature and operating temperature on crystalline phase, grain size, electronic conduction activation energy, and gas sensing response toward carbon monoxide (CO) and methane (CH₄) was studied. Pure anatase phase was found with crystallization temperatures up to 800 °C, however, rutile began to form by 900 °C. Grain size increased with increasing calcination temperature. Activation energy was dependent on crystallite size and phase. Sensing response toward CO and CH₄ was dependent on both calcination and operating temperatures. Films crystallized at 650 °C and operated at 450 °C showed the best selectivity toward CO.     

Preparation and characterization of TiO2 nanosponge

A procedure was developed to coat functionalized polystyrene spheres with well-defined layer of amorphous titanium dioxide. The core–shell particles can be turned into TiO₂ nanosponge by calcining the dried particles in a furnace. The phase transformation temperature of TiO₂ hybrid microspheres from anatase to rutile was increased by about 200 °C due to the blocking function of the calcined polymer remainder.     

Structural, electrochemical and optical comparisons of tungsten oxide coatings derived from tungsten powder-based sols

Tungsten trioxide (WO₃) electrochromic coatings have been formed on indium tin oxide-coated glass substrates by aqueous routes. Coating sols are obtained by dissolving tungsten powder in acetylated (APTA) or plain peroxotungstic acid (PTA) solutions. The structural evolution and electrochromic performance of the coatings as a function of calcination temperature (250 °C and 400 °C) have been reported. Differential scanning calorimetry and X-ray diffraction have shown that amorphous WO₃ films are formed after calcination at 250 °C for both processing routes; however, the coatings that calcined at 400 °C were crystalline in both cases. The calcination temperature-dependent crystallinity of the coatings results in differences in optical properties of the coatings. Higher coloration efficiencies can be achieved with amorphous coatings than could be seen in the crystalline coatings. The transmittance values (at 800 nm) in the colored state are 35% and 56% for 250 °C and 400 °C-calcined coatings, respectively. The electrochemical properties are more significantly influenced by the method of sol preparation. The ion storage capacities designating the electrochemical properties are found in the range of 1.62–2.74 × 10^− 3 (mC cm^− 2) for APTA coatings; and 0.35–1.62 × 10^− 3 (mC cm^− 2) for PTA coatings. As a result, a correlation between the microstructure and the electrochromic performance has been established.     

Trimethylamine sensing properties of nano-LaFeO3 prepared using solid-state reaction in the presence of PEG400

LaFeO₃ precursors are prepared using solid-state reaction in the presence of PEG400, and then LaFeO₃ nano-powders are obtained through heating these precursors under different conditions. Eventually, LaFeO₃ thick film sensors are fabricated by using LaFeO₃ nano-materials as sensing materials. The phase composition and morphology of particles in these materials are characterized through X-ray diffraction (XRD) and transmission electron microscopy (TEM), respectively. The XRD analysis results reveal that LaFeO₃ can be obtained by heating at 400–900 °C. TEM images manifest that the average particle sizes increase with heating temperature increasing, the particle sizes are in the range of 50–80 nm when heating temperature is increased to 800 °C. Furthermore, the influence of the heating duration and the heating temperature on the gas-sensing properties of the sensors based on LaFeO₃ nano-materials is also investigated in this work. The sensitivities to several organic gases, such as (CH₃)₃N and (CH₃)₂CO are studied. It is found that the sensor based on LaFeO₃ nano-material (800 °C, 2 h) exhibits best performance in all sensors investigated in this work. In detail, the sensitivities of the sensor based on LaFeO₃ nano-material (800 °C, 2 h) to 1000 and 0.001 ppm (CH₃)₃N at 208 °C are as high as 2553 and 1.6, respectively; and the response time and recovery time for 10 ppm trimethylamine are 8 and 50 s, respectively.     

Synthesis and photoluminescence properties of CaSiO3:Eu spheres prepared by the reverse micelles soft template

We report here the successful synthesis of CaSiO₃:Eu³⁺ spheres using the reverse micelles soft template. The influence of the calcination temperature on the shape, crystallization and photoluminescence properties of the prepared spheres was investigated by DTA-TG, XRD, IR, SEM and PL. The results showed that the temperature of crystallization (from amorphous phase to β-CaSiO₃) is 668 °C. The temperature of phase transition (from β-CaSiO₃ to α-CaSiO₃) is 790 °C. The average size of CaSiO₃:Eu³⁺ spheres calcined at 700 °C was about 350 nm. The radiation was dominated by the red emission peak at 613 nm and the highest emission intensity was observed when the spheres were calcined at 700 °C. When calcined at 800 °C, the spheres are almost cracked and melted down, due to the high temperature.     

Effect of crystallite size and calcination temperature on the thermal stability of single nanocrystalline chromium oxide: expressed by novel correlation

   The thermal reaction of chromium acetylacetonate in various organic solvents at 300 °C for 2 h yielded an amorphous product. Single nanocrystalline chromium oxide was obtained after being calcined at 300 °C for 1 h. The crystallite size of product is in the range of 16–26 nm. In this work, the thermal stability of product was given by BET/BET₀. It was found that the crystals of large crystallite size show higher thermal stability than the crystals of small crystallite size. Thermal stability of chromium oxide can be presented by the correlation of the BET surface area after calcination, crystallite size of as-synthesized product and calcination temperature (500–900 °C) as shown below.

Preparation of Barium Strontium Titanate Ceramic by Sol-Gel Method and Microwave Sintering

The (Ba,Sr)TiO₃ amorphous gel was prepared by sol-gel process and calcined in the 2.45-GHz multimode microwave furnace to synthesize (Ba,Sr)TiO₃ nanopowder. The calcination temperature of the (Ba,Sr)TiO₃ ceramic powders that convert the material into prevoskite phase can be reduced from 1100°C to 900°C, the nanopowder displays the highest sinterability. Using a new kind of insulator materials made of MgAl₂O₄–LaCrO₃, the crack-free and dense (Ba_0.80Sr_0.20)TiO₃ ceramics with fine grain size (<1 µm) were prepared by microwave sintering at 1310°C for 15 min. The fine (Ba,Sr)TiO₃ ceramics sintered by microwave sintering technique display lower dielectric loss than that of conventional samples, indicating a reduction of the influence of defects with the microwave process.     

Phase formation and dielectric properties of Ba<Subscript>0.5</Subscript>Sr<Subscript>0.5</Subscript>TiO<Subscript>3</Subscript> by slow injection sol–gel technique

A simple sol–gel process incorporating slow precursor injection technique was employed to synthesize homogeneous Ba_0.5Sr_0.5TiO₃ nano powders. The Ba_0.5Sr_0.5TiO₃ samples were subjected to calcination temperatures from 600 to 1,100 °C and sintering temperatures from 1,250 to 1,350 °C for the study of phase formation, crystallite size, particle distribution, and dielectric properties. Single phase Ba_0.5Sr_0.5TiO₃ with a cubic perovskite structure was successfully synthesized after calcination at 800 °C. The average size of the nano particles is 42 nm with a narrow size distribution, and a standard deviation of 10%. The highest values recorded within the investigated range for dielectric constant, and dielectric loss measured at 1 kHz are 1,164 and 0.063, respectively, for Ba_0.5Sr_0.5TiO₃ pellets calcined at 800 °C and sintered at 1,350 °C. Leakage current density measured at 5 V for the Ba_0.5Sr_0.5TiO₃ pellet was found to be 49.4 pA/cm².     

Preparation and properties of YAG nano-sized powder from different precipitating agent

Yttrium aluminum garnet (YAG) nano-sized powder was prepared by co-precipitation method assisted by ultrasonic display using different precipitating agent. The precursor powder can crystallize into YAG phase after being calcined at 900 °C for 2 h, because of the uniformly distribution of Al and Y elements. DTA–TG and IR measurements on the precursor powder obtained were performed to improve the synthesis process. X-ray diffraction was conducted on the powder calcined at different temperatures to investigate the effect of reaction conditions on particle size and phase composition. The powder about 15 nm in diameter with pure YAG was obtained at lower temperature (900 °C) by controlling the pH value, precipitating agent and doping agent.     

Photoluminescence from terbium doped silica-titania prepared by a sol-gel method

Terbium doped (0.5 at.%) TiO₂-SiO₂ (30%/70%) was prepared by a sol-gel method. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) were used to characterize the powder calcined at two different temperatures. At a low temperature of 550 °C an amorphous phase was obtained, but at a higher temperature of 1000 °C, the anatase TiO₂ phase was crystallized in the amorphous SiO₂ phase. Green photoluminescence from ultraviolet excitation was detected after heating to either temperature, but the amorphous sample heated to 550 °C exhibited a higher intensity. X-ray diffraction and photoluminescence excitation data are discussed to explain these observations.     

Research on the hot deformation behavior and graphite morphology of spheroidal graphite cast iron at high strain rate

The hot deformation behavior of spheroidal graphite cast iron (SGCI) was investigated quantitatively from 600 °C to 950 °C at high strain rate of 10 s⁻¹ by compression tests on a Gleeble-1500 simulator. The results show that the peak strain increases gradually with increasing deformation temperatures in the range of 600–800 °C and decreases when the temperature is raised to 800 °C and above. The optimum deformation temperature range is determined at 700–900 °C. The graphite particles become spindles or flakes after deformation, even some graphite collapse in the compressed specimens with about 0.7 peak strains. The graphite area fraction decreases as the temperature increases, at the same time, the high peak strain promotes the dissolving of carbon.     

Formation and transformation of ZnTiO3 prepared by sputtering process

Zinc titanate (ZnTiO₃) films were prepared using RF magnetron sputtering at substrate temperatures ranging from 30 to 400 °C. Subsequent annealing of the as-deposited films was performed at temperatures ranging from 600 to 900 °C. It was found that all as-deposited films were amorphous, as confirmed by XRD. This was further confirmed by the onset of crystallization that took place at annealing temperatures 600 °C. The phase transformation for the as-deposited films and annealed films were investigated in this study. The results revealed that pure ZnTiO₃ (hexagonal phase) can exist, and was obtained at temperatures between 700 and 800 °C. However, it was found that decomposition from hexagonal ZnTiO₃ to cubic Zn₂TiO₄ and rutile TiO₂ took place with a further increase in temperature to 900 °C.     

Low temperature mullite crystallization in Al- and Si-alkoxide derived homogeneous gels

A homogeneous mullite gel (3:2 Al₂O₃:SiO₂ molar ratio) was made from tetraethoxysilane (TEOS) and aluminum isopropoxide (Al(OCH(CH₃)₂)₃) at room temperature within a relatively quick three days. Mullite was the only crystalline phase to form during calcination; metastable phases like aluminosilicate spinel did not appear. When heated at 20 °C/min, crystalline mullite (65 mol% Al₂O₃) started forming at 575 °C and reached 27 wt.% by 900 °C. Major crystallization occurred at ~ 1000 °C with a concurrent increase of Al₂O₃ concentration (68–69 mol%) in the mullite phase. The alumina content decreased towards stoichiometric (3Al₂O₃·2SiO₂) mullite at even higher temperatures. When the gel remained in the amorphous state, low temperature preheating significantly improved crystallization at higher temperatures. After preheating at 425 °C for 24 h, 78 wt.% of the final product was crystallized when it was subsequently annealed at 750 °C for 5 min. Only 20 wt.% crystallized without preheating.     

Preparation of nanosized perovskite LaNiO3 powder via amorphous heteronuclear complex precursor

Nanosized lanthanum nickel oxide powder LaNiO₃ with perovskite structure was successfully synthesized at a relatively low calcination temperature by using an amorphous heteronuclear complex LaNi(DTPA) · 6H₂O as a precursor. The precursor decomposed completely into nickel oxide above 600°C based on the DTA and TGA results. XRD demonstrated that nanosized LaNiO₃ crystalline powder with pure perovskite structure was obtained after the calcination temperature increased to 700°C. The effects of calcination time and temperature were also examined by XRD and TEM. The results indicated the grain size and the crystal size of LaNiO₃ increased with the calcination temperature from 600°C to 900°C, and were less influenced by the heat-treatment time. The electrical resistivity of the powder decreased when the calcination temperature increased. It can be concluded that it is a useful way to synthesize nanosized perovskite oxides using an amorphous complex as a precursor, and this method can be easily quantitatively controlled.     

Oxidation characteristics of Ti3AlC2 over the temperature range 500–900 °C

Titanium aluminium carbide (Ti₃AlC₂) displays a unique combination of characteristics of both metals and ceramics coupled with an unusual combination of mechanical, electrical and thermal properties. In this research, the oxidation characteristics of Ti₃AlC₂ over the temperature range 500–900 °C were studied by synchrotron radiation diffraction (SRD) and secondary-ion mass spectrometry (SIMS) experiments which provided elemental and phase compositional depth profiles over this range. Evidence for the outward diffusion of Al during oxidation was shown for the first time by the complementary SIMS results, which suggested the existence of amorphous Al or aluminium oxide at low temperature oxidation. At 500 °C, only anatase-TiO₂ was detected by SRD in addition to the parent Ti₃AlC₂. Transformation of anatase to rutile was observed at 600 °C and was completed by 900 °C. The crystalline phase α-Al₂O₃ was detected at 900 °C but not at lower temperatures.     

Influence of calcination temperature of kaolin on the structure and properties of final geopolymer

In this paper, the structure of two types of metakaolins from kaolin calcined at 800 and 900 °C, respectively, and the obtained geopolymer were systematically characterized. It was found that calcination temperature had little effect on the environment of silicon atoms but had great effect on that of aluminum ones. ²⁷Al NMR analysis showed that tetrahedral aluminums in the metakaolin from kaolin calcined at 800 and 900 °C were in different environment, of the type AlQ₃(3Si) and AlQ₄(4Si), respectively, leading to different environment of aluminum atoms in the resulted geopolymer. Aluminum atoms in the geopolymer based on metakaolin from kaolin calcined at 800 °C were in the types of tetrahedral and octahedral, and silicon atoms were in the types of tetrahedral Q₄(3Al) together with a small amount of Q₄(0Al). However, geopolymer based on metakaolin from kaolin calcined at 900 °C consisted of Q₄(4Si) unit aluminum and Q₄(3Al) unit silicon. The results revealed that the calcination temperature had a great effect on environment of the aluminum atoms of the metakaolin, thus led to the different structure and properties including mechanical strength and thermal conductivity of the post obtained geopolymer.     

Spark-plasma-sintering temperature dependence of structural and piezoelectric properties of BNT–BT<Subscript>0.08</Subscript> nanostructured ceramics

(Bi_0.5Na_0.5)TiO₃ doped with 8 mol% BaTiO₃ powder prepared by sol–gel was compacted and sintered by spark-plasma-sintering method. The influence of spark-plasma-sintering temperature on the densification and piezoelectric properties of these ceramics was studied. Starting from BNT–BT_0.08 powder gel with a microstructure consisting of particles with average size of 50 nm, ceramics with grain size of 60–90 nm and density of about 98.9–99.6% of the theoretical density were obtained by spark-plasma-sintering at 800–900 °C. Increasing the sintering temperature by SPS from 800 to 900 °C lead to the increase of d ₃₃, k _p, ε₃₃^T and, decrease of Q _m. Typical d ₃₃ and k _p values of BNT–BT_0.08 ceramics sintered by spark-plasma-sintering at 900 °C were 8 and 0.029, respectively.     

Synthesis and photocatalytic activity of nanocrystalline TiO2-SrO composite powders under visible light irradiation

Nano-sized homogeneously distributed TiO₂-20, -40, -60 wt.% SrO composite powders were successfully synthesized by a sol-gel method. The as-received amorphous TiO₂—20 wt.% SrO composite powders were crystallized with anatase TiO₂ at around 750 °C. As calcination temperatures increased, the anatase TiO₂ crystalline phase was transformed to rutile TiO₂ at about 900 °C, whereas nano-sized, squarish SrTiO₃ phase was detected. The peaks obtained after calcining at 1050 °C mainly exhibited the rutile TiO₂ and SrTiO₃ phases. However, a small number of SrO₂ peaks were also detected. For the comparison of photocatalytic activity depending on light sources, TiO₂-SrO composite powders were tested in phenol degradation. TiO₂-60 wt.% SrO composite powder showed good visible light photoactivity for the photo-oxidation of phenol.     

Preparation and characterization of ultrafine alumina via sol–gel polymeric route

Ultrafine alumina powder was prepared through resin formation between urea and formaldehyde. Aluminium stearate soap was introduced during resin preparation. Ethylene glycol was used to terminate the thermosetting reaction. Calcination of the product was carried out at 700, 1000, 1100, 1300 and 1400 °C to obtain aluminium oxide.IR and Raman spectroscopic analysis indicated the occupation of Al³⁺ at different sites in the polymer network (CO, NH₂, CO, NH, and CH₂OH).X-ray diffraction of powder calcined at 1000 °C revealed the presence of a mixture of α- and θ-alumina together, while a mixture of α- and β-alumina phases were obtained on calcination at 1400 °C. Transmission electron microscope (TEM) examination of the powder fired at 700 °C showed uniform grains in the form of clusters with average size between 22.02 and 30.5 nm. Clusters are multi-particles as evident from the electron diffraction pattern. Crystallite size of alumina powder calcined at 1000 °C was found to be ≈25.67 nm, while that of powder calcined at1400 °C was ≈30.52 nm. The calculated specific surface area of alumina powder calcined at 1000 °C was 59.17 m² g⁻¹, while that calcined at 1400 °C was 49.77 m² g⁻¹.     

Preparation and characterization of Y3Al5O12 (YAG) nano-powder by co-precipitation method

YAG precursor was synthesized by a co-precipitation method from a mixed solution of aluminum and yttrium nitrates with aqueous ammonia as the precipitator. The structure, phase evolution and morphology of YAG precursor and the sintered powders were studied by means of IR, TG/DTA, XRD, TEM methods. It was found the precursor with approximate composition of Al(OH)₃·0.3[Y₂(OH)₅·(NO₃)₂·2H₂O] directly transformed to pure-YAG phase at 800 °C and no intermediate phases were detected. YAG nanocrystalline powders from sintering the precursor at different temperatures were less-aggregated and the diameters of the grains were about 40-100 nm. BET surface area of the particles decreased with increase of calcination temperature and the powder sintered at 800 °C can be used for fabrication of transparent YAG ceramics.