Effects of mechanical properties of polymer on ceramic-polymer composite thick films fabricated by aerosol deposition

ABSTRACT: Two types of ceramic-polymer composite thick films were deposited on Cu substrates by an aerosol deposition process, and their properties were investigated to fabricate optimized ceramic-based polymer composite thick films for application onto integrated substrates with the advantage of plasticity. When polymers with different mechanical properties, such as polyimide (PI) and poly(methyl methacrylate) (PMMA), are used as starting powders together with α-Al2O3 powder, two types of composite films are formed with different characteristics - surface morphologies, deposition rates, and crystallite size of α-Al2O3. Through the results of micro-Vickers hardness testing, it was confirmed that the mechanical properties of the polymer itself are associated with the performances of the ceramic-polymer composite films. To support and explain these results, the microstructures of the two types of polymer powders were observed after planetary milling and an additional modeling test was carried out. As a result, we could conclude that the PMMA powder is distorted by the impact of the Al2O3 powder, so that the resulting Al2O3-PMMA composite film had a very small amount of PMMA and a low deposition rate. In contrast, when using PI powder, the Al2O3-PI composite film had a high deposition rate due to the cracking of PI particles. Consequently, it was revealed that the mechanical properties of polymers have a considerable effect on the properties of the resulting ceramic-polymer composite thick films.     

A comparative study on UV light activated porous TiO2 and ZnO film sensors for gas sensing at room temperature

In order to find a new approach for screening the photoactivated gas sensing materials with high sensitivity, a comparative study was carried out. With the simple technique of screen printing, TiO₂ and ZnO were used to fabricate the UV light activated gas sensors which were applied at room temperature. To facilitate the simultaneous measurements of the current transients of the two materials, they were printed on the same alumina substrate. Compared with ZnO, TiO₂ exhibited a superior performance to ethanol and formaldehyde gases. It was found that the responses of TiO₂ increased with the concentration of test gas and amounted to 224 and 1700 to 100 ppm ethanol and formaldehyde gases, respectively, while the responses of ZnO to 100 ppm ethanol and formaldehyde gases were 0.14 and 1.5, respectively. The mechanism of such a huge difference between TiO₂ and ZnO was discussed in detail. Furthermore, it is suggested that metal oxide semiconductor with lower photo-to-dark current ratio can achieve higher photoactivated gas sensitivity.     

Effect of structural defects,surface roughness on sensing properties of Al doped ZnO thin films deposited by chemical spray pyrolysis technique

In this article, the gas sensing properties of Al-doped ZnO thin films have been reported where the nanocrystalline ZnO based thin films were well deposited by a simple and inexpensive ‘chemical spray pyrolysis (CSP)’ technique. Films have been found to be uniform, pinhole free and well adherent to the substrate. The morphology, structures, and surface roughness of the deposited Al-doped ZnO thin films were studied by various types of characterization techniques. In addition, the authors have observed that the sensor response and selectivity towards CO gas is improved by the Al doping at a low operating temperature. XRD results showed that the obtained films are nanocrystalline in nature with hexagonal wurtzite phase. Further, the annealed films were used for detection of CO in the air and maximum response was observed at 175 °C. The improvement in sensor response of Al-doped ZnO thin films to CO gas attributed to the defect chemistry, crystallite size and surface roughness.     

Direct synthesis of ultrafine tetragonal BaTiO3 nanoparticles at room temperature

A large quantity of ultrafine tetragonal barium titanate (BaTiO3) nanoparticles is directly synthesized at room temperature. The crystalline form and grain size are checked by both X-ray diffraction and transmission electron microscopy. The results revealed that the perovskite nanoparticles as fine as 7 nm have been synthesized. The phase transition of the as-prepared nanoparticles is investigated by the temperature-dependent Raman spectrum and shows the similar tendency to that of bulk BaTiO3 materials. It is confirmed that the nanoparticles have tetragonal phase at room temperature.     

Electrochemical behaviors of graphene-ZnO and graphene-SnO2 composite films for supercapacitors

Graphene, graphene-ZnO and graphene-SnO₂ films were successfully synthesized and used as electrode materials for electrochemical supercapacitors, respectively. The screen-printing approach was employed to fabricate graphene film on graphite substrate while the ZnO and SnO₂ were deposited on graphene films by ultrasonic spray pyrolysis. The electrochemical performances of these electrodes were comparatively analyzed through electrochemical impedance spectrometry, cyclic voltammetry and chronopotentiometry tests. The results showed that the incorporation of ZnO or SnO₂ improved the capacitive performance of graphene electrode. Graphene-ZnO composite electrode exhibited higher capacitance value (61.7 F/g) and maximum power density (4.8 kW/kg) as compared with graphene-SnO₂ and pure graphene electrodes.     

Influence of flame conditions on the dispersion of Pd on the flame spray-derived Pd/TiO2 nanoparticles

The Pd/TiO₂ nanoparticles containing 5 wt.% Pd were synthesized by one-step flame spray pyrolysis (FSP) under different flame conditions. As revealed by both X-ray diffraction (XRD) and transmission electron microscopy (TEM) results, the average particle sizes of Pd/TiO₂ were increased from 9.7 to 24.6 nm with increasing the precursor concentration and the feed flow rate as well as reduction of the O₂ dispersing gas during FSP synthesis. Although the BET surface area and %anatase phase content were decreased with increasing Pd/TiO₂ particle size, %Pd dispersion as determined from the amounts of CO chemisorption were higher on the larger size FSP-made Pd/TiO₂ nanoparticles. It is suggested that the shorter residence time in flame and/or the lower combustion energy (enthalpy density) resulted in more coverage of Pd surface by the formation of Ti-O groups, rendering lower CO chemisorption ability of the smaller size Pd/TiO₂.     

Characterisation of PEO-Al2O3 composite polymer electrolytes

The behaviour of PEO₈LiClO₄ with different quantities of α-Al₂O₃ or γ-Al₂O₃ was investigated using DSC, AC conductivity and ⁷Li NMR experiments. DSC results showed that the presence of the filler does not change the glass transition temperature of the electrolyte but, on the other hand, modifies the quantity of its crystalline phase. From the AC impedance measurements, it was observed that the sample with the highest conductivity at room temperature is PEO₈LiClO₄ 5.3 wt.% α-Al₂O₃. The change in the quantity of crystalline phase cannot alone explain the conductivity data, and it is suggested that the space charge contribution in the interphase of the filler particles and the polymeric chains influences the behaviour of the samples. The ⁷Li NMR results showed that line width narrowing begins at temperatures close to T_g. From the hydrogen decoupling experiments it was possible to estimate the LiH average distances as 2.7 Å. The LiLi distance was calculated as being between 2.6 and 3.5 Å depending on the number of near neighbours lithium nuclei used in the model.     

Room temperature ammonia sensing based on graphene oxide integrated flexible polyvinylidenefluoride/cerium oxide nanocomposite films

ABSTRACT

In this work, novel room temperature (RT) ammonia (NH₃) sensors comprising graphene oxide (GO) integrated polyvinylidenefluoride (PVDF)/cerium oxide (CeO₂) nanocomposite films have been prepared via simple solution casting technique. The structural and morphological characteristics of flexible tertiary PVDF/CeO₂/GO nanocomposite films have been investigated using various analytical techniques and their NH₃ gas-sensing performance was evaluated at RT and the relevant sensing mechanism was established. The flexible PVDF/CeO₂/GO nanocomposite films responded strongly to NH₃ gas with enhanced gas sensing properties at RT as compared with various other volatile organic compounds (VOCs) such as acetone, ethanol, formaldehyde and toluene.     

Effects of Co dopants and flame conditions on the formation of Co/ZrO2 nanoparticles by flame spray pyrolysis and their catalytic properties in CO hydrogenation

The Co/ZrO₂ catalysts with various Co loadings (5–10 wt.%) were prepared by one-step flame spray pyrolysis (FSP) under different flame conditions. As revealed by XRD and TEM, all the resulting Co/ZrO₂ nanoparticles were composed of single-crystalline particles exhibiting the characteristic tetragonal structure of ZrO₂. Varying the amount of Co dopants during FSP synthesis did not alter the primary particle size of ZrO₂ which was determined to be ca. 14 nm. On the other hand, increasing precursor feed rate from 3 to 8 ml/min resulted in an increase of ZrO₂ crystallite size from 10 to 19 nm. The higher precursor feed rate produced higher enthalpy of flame and longer residence times, which increased coalescence and sintering of the particles. Compared to the Co/ZrO₂ prepared by conventional impregnation method, the catalytic activities of the FSP-made catalysts were much higher. Moreover, the hydrogenation rates of the FSP-made Co/ZrO₂ catalysts were increased with increasing Co loading and precursor feed rate. According to H₂ chemisorption and H₂ temperature program reduction results, the improvement of catalytic activity and C₂–C₆ selectivities of the FSP-made catalysts in the CO hydrogenation was attributed to the higher number of Co metal active sites and lower interaction between Co/CoO and ZrO₂ support obtained via the FSP synthesis.     

Optical and microstructural properties of ZnO/TiO2 nanolaminates prepared by atomic layer deposition

ZnO/TiO₂ nanolaminates were grown on Si (100) and quartz substrates by atomic layer deposition at 200°C using diethylzinc, titanium isopropoxide, and deionized water as precursors. All prepared multilayers are nominally 50 nm thick with a varying number of alternating TiO₂ and ZnO layers. Sample thickness and ellipsometric spectra were measured using a spectroscopic ellipsometer, and the parameters determined by computer simulation matched with the experimental results well. The effect of nanolaminate structure on the optical transmittance is investigated using an ultraviolet–visible-near-infrared spectrometer. The data from X-ray diffraction spectra suggest that layer growth appears to be substrate sensitive and film thickness also has an influence on the crystallization of films. High-resolution transmission electron microscopy images show clear lattice spacing of ZnO in nanolaminates, indicating that ZnO layers are polycrystalline with preferred (002) orientation while TiO₂ layers are amorphous.     

Characteristics and catalytic properties of Pt–Sn/Al2O3 nanoparticles synthesized by one-step flame spray pyrolysis in the dehydrogenation of propane

The Pt–Sn/Al₂O₃ catalysts with 0.3 wt% Pt and 0.5–1.5 wt% Sn loading were prepared by one-step flame spray pyrolysis (FSP). Unlike the catalysts prepared by conventional impregnation method, the FSP-derived catalysts were composed of single-crystalline γ-alumina particles with the as-prepared primary particle size of 10–18 nm and contained only large pores. The FSP catalysts exhibited superior catalytic activity and better stability than the ones made by impregnation in the dehydrogenation of propane, while they did not alter the selectivity to propylene (in all cases, propylene selectivity ≥96%). The presence of large pores in the flame-made catalysts not only facilitated diffusion of the reactants and products but could also lessen the amount of carbon deposited during reactions. As revealed by CO chemisorption, transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS), the metal particles appeared to be partially covered by the alumina matrix (Al–O) due to the simultaneous formation of particles during FSP synthesis. Such phenomena, however, were shown to result in the formation of active Pt–Sn ensembles for propane dehydrogenation as shown by higher turnover frequencies (TOFs).     

Preparation and characterization of CeO2/TiO2 nanoparticles by flame spray pyrolysis

Nanocrystalline TiO₂, CeO₂ and CeO₂-doped TiO₂ have been successfully prepared by one-step flame spray pyrolysis (FSP). Resulting powders were characterized with X-ray diffraction (XRD), N₂-physisorption, Transmission Electron Microscopy (TEM) and UV-Vis spectrophotometry. The TiO₂ and CeO₂-doped TiO₂ nanopowders were composed of single-crystalline spherical particles with as-prepared primary particle size of 10-13 nm for Ce doping concentrations of 5-50 at%, while square-shape particles with average size around 9 nm were only observed from flame-made CeO₂. The adsorption edge of resulting powder was shifted from 388 to 467 nm as the Ce content increased from 0 to 30 at% and there was an optimal Ce content in association with the maximum absorbance. This effect is due to the insertion of Ce^3+/4+ in the TiO₂ matrix, which generated an n-type impurity band.     

Role of ZnO in Cu/ZnO/Al2O3 catalyst for hydrogenolysis of butyl butyrate

For hydrogenolysis of butyl butyrate (BB), a series of Cu/ZnO/Al₂O₃ catalysts with different metal compositions were prepared, and characterized by N₂O chemisorption for measuring Cu surface area and by chromatographic experiment for determining the heat of BB adsorption. As a result, the presence of ZnO in Cu-based catalysts was found to enhance the catalytic activity of Cu due to dual function of ZnO. The Cu surface area was linearly correlated with the butanol productivity, demonstrating that ZnO exerts the structural function in Cu/ZnO/Al₂O₃ catalysts. Additionally, the role of ZnO as a chemical contributor was revealed such that its presence leads to lower activation energy of the surface reaction, thus resulting in higher Cu catalytic activity obtained at a low temperature such as 200 °C. Consequently, optimizing the Cu/Zn ratio in Cu/ZnO/Al₂O₃ catalyst is required to tune its structural and chemical characteristics of Cu metals, and thus to obtain a higher activity on the hydrogenolysis reaction.     

Gas sensing properties of multiple networked GaN/WO3 core-shell nanowire sensors

GaN nanowires and GaN-core/WO₃-shell nanowires were synthesized by the thermal evaporation of GaN powders followed by the sputter-deposition of WO₃ and their gas sensing properties were examined. The multiple networked pristine GaN nanowire sensors showed responses of approximately 125%, 140%, 146%, 159%, and 183% to 1, 2, 3, 4, and 5 ppm NO₂ gases, respectively. These responses are comparable to those obtained previously using metal oxide semiconductor one-dimensional nanostructure sensors. The responses of the nanowires to 1, 2, 3, 4, and 5 ppm NO₂ gases were improved 1.3, 1.4, 1.6, 1.7 and 1.8 fold, respectively, further through the encapsulation of GaN nanowires with a WO₃ thin film. The improvement in the response of GaN nanowires to NO₂ gas by encapsulation is attributed to the modulation of electron transport at GaN–WO₃ heterojunction. The electron transport in the core-shell nanowires is modulated by the heterojunction with an adjustable energy barrier height, resulting in an enhanced sensing property of the core-shell nanostructures.     

Preparation of ZrO2/Gd2O3 composite ceramic materials by coprecipitation method

High burnup is a goal for further development of advanced nuclear power in the future. However, along with the increase of burnup, it becomes more diffidult to control reactor reactivity, which affects the operation safety of the nuclear reactor. Al₂O₃/B₄C burnable poison materials widely used in pressurized water reactor currently will not meet the requirements of burnable poison materials in high burnup nuclear power. Because of the better performance of ZrO₂/Gd₂O₃ burnable poison materials than that of Al₂O₃/B₄C, this paper studies the preparation of ZrO₂/Gd₂O₃ composite ceramic materials by the coprecipitation method. The experimental results show that at the sintering temperature of 1500–1650 °C, ZrO₂/Gd₂O₃ composite ceramic grains are small, compact and uniform with the generation of homogeneous solid solution. At 1600 °C, ZrO₂–10%Gd₂O₃ has the highest density and mechanical strength.     

Pd/C-catalyzed practical degradation of PCBs at room temperature

The catalytic degradation method of polychlorinated biphenyls (PCBs) using the palladium on activated carbon–triethylamine (Pd/C–Et₃N) system under ambient hydrogen pressure and temperature was developed. Aroclor^® 1254, Aroclor^® 1248, 10% Aroclor^® 1254 in paraffin oil and PCBs from capacitor could be completely dechlorinated to afford biphenyl and Et₃N·HCl. Fifteen pure PCB congeners, including the highly toxic co-planar PCBs, were smoothly dechlorinated to biphenyl within 1 or 2 h using 10% Pd/C (10% of substrate weight) and Et₃N (1.2 equiv. vs. Cl numbers). However, the dechlorination of the fully ortho-substituted PCB congeners was delayed and chlorine atoms on the ortho-positions still remained under the hydrogenation conditions, but these PCB congeners are only slightly present in the commercial PCB mixture. The Pd/C–Et₃N–H₂ system offers a simple, safe, and inexpensive degradation method of PCBs under mild reaction conditions.     

Effect of molar ratio of TiO2/SiO2 on the properties of particles synthesized by flame spray pyrolysis

Titania (TiO₂)–silica (SiO₂) nanoparticles were synthesized from sprayed droplets of a mixture of TEOS and TTIP by flame spray pyrolysis (FSP). The effect of molar ratio between TEOS and TTIP in the mixture on the particle properties such as particle morphology, average particle diameter, specific surface area, crystal structure, etc., were determined using TEM, XRD, BET, and FT-IR. A UV-spectrometer was also used to measure the absorption spectrum and the band gap energy of the product particles. As the molar ratio of TEOS/TTIP increased by increasing TEOS concentration at the fixed TTIP concentration, the average particle diameter of the mixed oxide nanoparticles increased with maintaining uniform dispersion between TiO₂ and SiO₂, and crystal structure was transformed from anatase to amorphous. The band gap energy of the TiO₂–SiO₂ nanoparticles increased with respect to the increase of the molar ratio due to the decrease of width of UV-absorption spectrum. Photocatalytic activity of TiO₂–SiO₂ composite particles decreased with the concentration of TEOS.     

Improvement of the physical properties of ZnO/CdTe core-shell nanowire arrays by CdCl2 heat treatment for solar cells

CdTe is an important compound semiconductor for solar cells, and its use in nanowire-based heterostructures may become a critical requirement, owing to the potential scarcity of tellurium. The effects of the CdCl₂ heat treatment are investigated on the physical properties of vertically aligned ZnO/CdTe core-shell nanowire arrays grown by combining chemical bath deposition with close space sublimation. It is found that recrystallization phenomena are induced by the CdCl₂ heat treatment in the CdTe shell composed of nanograins: its crystallinity is improved while grain growth and texture randomization occur. The presence of a tellurium crystalline phase that may decorate grain boundaries is also revealed. The CdCl₂ heat treatment further favors the chlorine doping of the CdTe shell with the formation of chlorine A-centers and can result in the passivation of grain boundaries. The absorption properties of ZnO/CdTe core-shell nanowire arrays are highly efficient, and more than 80% of the incident light can be absorbed in the spectral range of the solar irradiance. The resulting photovoltaic properties of solar cells made from ZnO/CdTe core-shell nanowire arrays covered with CuSCN/Au back-side contact are also improved after the CdCl₂ heat treatment. However, recombination and trap phenomena are expected to operate, and the collection of the holes that are mainly photo-generated in the CdTe shell from the CuSCN/Au back-side contact is presumably identified as the main critical point in these solar cells.     

Synthesis of Fe2O3 nanoparticles for nitrogen dioxide gas sensing applications

Iron (III) oxide, Fe₂O₃, nanoparticles of approximately 40 nm diameter were synthesized by sol–gel method and their nitrogen dioxide adsorption and desorption kinetics were investigated by custom fabricated gas sensor unit. The morphology and crystal structure of Fe₂O₃ nanoparticles were studied by scanning electron microscopy (SEM), field emission scanning electron microscopy (FESEM) and X-ray diffraction (XRD) respectively. The roughness of film surface was investigated by atomic force microscopy (AFM). Relative sensitivity of Fe₂O₃ nanoparticles for NO₂ sensor was determined by electrical resistance measurements. Our reproducible experimental results show that Fe₂O₃ nanoparticles have a great potential for nitrogen dioxide sensing applications operating at a temperature of 200 °C.     

Synthesis and high sensing properties of a single Pd-doped SnO2 nanoribbon

Monocrystal SnO₂ and Pd-SnO₂ nanoribbons have been successfully synthesized by thermal evaporation, and novel ethanol sensors based on a single Pd-SnO₂ nanoribbon and a single SnO₂ nanoribbon were fabricated. The sensing properties of SnO₂ nanoribbon (SnO₂ NB) and Pd-doped SnO₂ nanoribbon (Pd-SnO₂ NB) sensors were investigated. The results indicated that the SnO₂ NB showed a high sensitivity to ethanol and the Pd-SnO₂ NB has a much higher sensitivity of 4.3 at 1,000 ppm of ethanol at 230°C, which is the highest sensitivity for a SnO₂-based NB. Pd-SnO₂ NB can detect ethanol in a wide range of concentration (1 ~ 1,000 ppm) with a relatively quick response (recovery) time of 8 s (9 s) at a temperature from 100°C to 300°C. In the meantime, the sensing capabilities of the Pd-SnO₂ NB under 1 ppm of ethanol at 230°C will help to promote the sensitivity of a single nanoribbon sensor. Excellent performances of such a sensor make it a promising candidate for a device design toward ever-shrinking dimensions because a single nanoribbon device is easily integrated in the electronic devices.