Nanostructured Perovskites for Catalytic Combustion

Until recently, for reducing the environment pollution, the combustion catalysts based on noble metals was considered the most active, practically irreplaceable. Their high cost, high thermal instability, high sensitivity to deactivation and to the attack of some harmful elements or compounds determine the intensification of studies to replace them with new cheaper and stable catalysts. Numerous experimental data from literature indicate that the semiconductive oxidic compounds can compete with combustion catalysts based on noble metals from catalytic activity standpoint. Recent studies led to the realization of remarkable catalytic activity at moderate (350–600°C) and high (over 600°C) temperatures at some oxidic perovskite compounds, which contain transition metals. In this study are presented a series of nanostructured oxidic compounds with perovskite structure, based on transition metals and synthesized by the precursor method of self-combustion, using polyvinyl alcohol as colloidal medium, for catalytic combustion of some volatile organic compounds at low (50–350°C) and moderate temperatures. The catalytic activity of the perovskite compounds in the total oxidation reactions of the gases is largely determined by the amount of weakly bound surface oxygen species which in turn depends on the presence of oxygen vacancies

Nanostructured Oxide Semiconductor Compounds with Possible Applications for Gas Sensors

Nanostructured oxide semiconductor compounds have gained a big importance, in basic and mostly in applicative researches, due to their unique properties, and their increased potential of utilization as sensors in various electronic and optoelectronic devices. The development of devices based on semiconductor materials as gas sensors has been visible during the recent years, due to their low manufacturing cost. Because the basic materials and the manufacturing processes are critical for gas sensors high performance, they need to be studied and capitalized in practice. Among the new technologies, the production of nanocrystalline materials and hybrid structures offer huge opportunities to improve sensitivity, selectivity and response time, as a consequence of the intensification of gas-sensor interaction. In this study, a series of nanostructured oxide semiconductor compounds with a spinel-type structure and perovskite, respectively, based on transition metals and synthesized by the sol-gel self-combustion method, with possible applications for resistive gas sensors, are presented

Influence of Substrate Temperature on the Properties of Ga Doped ZnO thin Films

Transparent conductive oxides, including single layer and multilayer structures, have been investigated in the last years, for transparent or semitransparent conducting electrode applications in optoelectronic devices and transparent electronics. Owing to their properties such as: high transmission in the visible region (>80%) and high electrical conductivity, they been used in applications like LEDs, laser diodes, solar cell, flat panel displays, thin films transistors etc Some alternative TCO materials including ZnO doped with III rd group elements (In, Al, Ga); TiO 2 doped with Ce, Nb, Fe; SnO 2 doped with F etc., have been investigated in the last years In order to use Ga doped ZnO thin films for different application it is very important to obtain layers with reproducible properties. Because the deposition parameters of each deposition technique can be varied, the properties of obtained thin films may differ significantly. Despite the progress made in the field, the researchers are still doing studies to establish the most convenient deposition parameters for processing Ga doped ZnO thin films with excellent optical and electrical properties. A large number of techniques were used in order to obtain ZnO:Ga thin films including pulsed laser deposition, reactive plasma deposition, RF Magnetron Sputtering, The aim of this paper is reporting some results regarding the influence of substrate temperature on the physical properties of ZnO:Ga thin films prepared by RF magnetron sputtering being analysed by XRD, SEM, Profilometry and two point method for electric properties. Experimental part Materials and methods The obtaining of thin films Ga doped ZnO thin films were deposited on glass substrates by RF Magnetron Sputtering using a 98 wt. % ZnO doped with 2 wt. % Ga 2 O 3 ceramic target of 40 mm diameter, keeping RF power constant at 50W. The substrate temperature was of 300K (sample 1), 423K (sample 2) and 523K (sample 3) respectively. The distance between target and substrate was maintained at 55 mm. For the thin film deposition, the work pressure was maintained at 1.4 mTorr in argon atmosphere. The deposition parameters are presented in table 1. Analysis methods and techniques Structural analyses of the studied films were carried out using a DRON 2 diffractometer with CuK α radiation (λ = 1.5418 A) as an X -ray source at 20 kV and 40 mA, in the 2θ =20-70° range. The films morphology was investigated by using scanning electron microscopy (SEM Tescan VEGA II LSH). The thickness of investigated films, measured using a DEKTAK profilometer, was found to be around of 150 nm. The electrical properties of studied samples were measured in a two points configuration, by using a sensitive Keithley model 6517 multimeter. For gas sensing measurements, the thin films were placed in a glass enclosure capable of controlling the different gas concentrations and working temperature. The resistance was measured in the presence of the tested gas. The gas sensing properties were investigated at various operating temperatures from 373K to 750K. As test gases were use

Structural, Electrical and Optical Properties of Pyrrolo[1,2-i][1,7] Phenanthroline-Based Organic Semiconductors

This work reports a study on structural, electrical and optical properties of some recently synthesized pyrrolo[1,2-i][1,7] phenanthrolines derivatives in thin films. The thin films were deposited onto glass substrates by spin coating technique, using chloroform as solvent. The obtained films exhibited a polycrystalline structure with an n–type semiconductor behavior after heat treatment in the temperature range 293–543 K, specific to each sample. The thermal activation energy lies between 0.68 and 0.78 eV, while the direct optical band gap values were found in the range 4.17–4.24 eV. The electrical and optical properties of the investigated organic semiconductor films were discussed in relation to microstructural properties, determined by the molecular structure. The investigated organic compounds are promising for applications in organic optoelectronics and nanoelectronics

Porous Nanostructured Gadolinium Aluminate for High-Sensitivity Humidity Sensors

This paper presents the synthesis of gadolinium aluminate (GdAlO3), an oxide compound with a perovskite structure, for applications as a capacitive and/or resistive humidity sensor. Gadolinium aluminate was synthesized by the sol-gel self-combustion method. This method allowed us to obtain a highly porous structure in which open pores prevail, a structure favorable to humidity sensors. Most of the materials studied as capacitive/resistive humidity sensors have significant sensitivities only with respect to one of these types of sensors. In the case of the studied gadolinium aluminate with p-type electric conductivity, the relative humidity of the air has a significant influence on both capacitive and resistive types of electric humidity sensors. The capacity increases about 10,000 times, and the resistance decreases about 8000 times as the relative humidity increases from 0 to 98%. The investigated gadolinium aluminate can be used successfully to obtain high-sensitivity capacitive and/or resistive humidity sensors

HUMIDITY-SENSITIVE ELECTRICAL RESISTIVITY OF MgFe 2 O 4 AND Mg 0.9 Sn 0.1 Fe 2 O 4 POROUS CERAMICS

Pure and Sn substituted Mg ferrites have been prepared by sol-gel autocombustion method. This method offers the advantage to prepare ultra-fine, homogeneous and reproducible ferrite powders using aqueous solutions of constituent ions (metal nitrates) salts. Sn ion partially substituted Mg ion to improve the humidity sensitivity of Mg polycrystalline ferrite. The phase composition and lattice parameters were determined by X-ray diffraction (XRD), and the effect of Sn substitution on the granular structure was investigated by scanning electron microscopy (SEM). We have found that Sn ions affect crystallite size, surface area and porosity. Sn ions assure a nanocrystalline structure of Mg 0.9 Sn 0.1 Fe 2 O 4 ferrite. The average grain size changes from 500 nm to 100 nm by partial substituting Mg with Sn ions in MgFe 2 O 4 . Also, Sn ions enhance the humidity sensitivity of Mg ferrite. For Sn substituted ferrite, the electrical resistivity decreases by about three orders of magnitude, from 10 9 Ω·cm to 10 6 Ω·cm when the relative humidity increases from 11% to 85%. These results show that the promoting effect of Sn ions on humidity sensitivity of Mg ferrite can be related with the changes in porosity and specific surface area. Further investigations are necessary to shorten response time to humidity variations

On the Electrical and Optical Properties Stability of P3HT Thin Films Sensitized with Nitromethane Ferric Chloride Solutions

The electrical and optical properties stability of poly(3-hexylthiophene) (P3HT) thin films sensitized with nitromethane ferric chloride (FeCl3) solution was investigated. The optical properties modifications were studied by spectrophotometry and ellipsometry. For electrical characterizations, electrical resistivity measurements were performed. In agreement with the observations of other authors, an important decrease in the electrical resistivity by six orders of magnitude was noticed. In addition, the repeatability and stability of this phenomenon were investigated over a few weeks after sensitization and during different cycles of heating and cooling, both in the dark and under illumination

Structural, Electrical and Optical Properties of Pyrrolo[1,2-<i>i</i>][1,7] Phenanthroline-Based Organic Semiconductors

This work reports a study on structural, electrical and optical properties of some recently synthesized pyrrolo[1,2-i][1,7] phenanthrolines derivatives in thin films. The thin films were deposited onto glass substrates by spin coating technique, using chloroform as solvent. The obtained films exhibited a polycrystalline structure with an n–type semiconductor behavior after heat treatment in the temperature range 293–543 K, specific to each sample. The thermal activation energy lies between 0.68 and 0.78 eV, while the direct optical band gap values were found in the range 4.17–4.24 eV. The electrical and optical properties of the investigated organic semiconductor films were discussed in relation to microstructural properties, determined by the molecular structure. The investigated organic compounds are promising for applications in organic optoelectronics and nanoelectronics

Studies of the Structure and Optical Properties of BaSrMgWO6 Thin Films Deposited by a Spin-Coating Method

Highly transparent thin films with the chemical formula BaSrMgWO6 were deposited by spin coating using a solution of nitrates of Ba, Sr, and Mg and ammonium paratungstate in dimethylformamide with a Ba:Sr:Mg:W ratio = 1:1:1:1. XRD, SEM, EDX, and XPS investigations evidenced that annealing at 800 &deg;C for 1 h results in an amorphous structure having a precipitate on its surface, and that supplementary annealing at 850 &deg;C for 45 min forms a nanocrystalline structure and dissolves a portion of the precipitates. A textured double perovskite cubic structure (61.9%) was found, decorated with tetragonal and cubic impurity phases (12.7%), such as BaO2, SrO2, and MgO, and an under-stoichiometric phase (24.4%) with the chemical formula Ba2&minus;(x+y) SrxMgyWO5. From transmittance measurements, the values of the optical band gap were estimated for the amorphous (Egdir = 5.21 eV, Egind = 3.85 eV) and nanocrystalline (Egdir = 4.69 eV, Egind = 3.77 eV) phases. The presence of a lattice disorder was indicated by the high Urbach energy values and weak absorption tail energies. A decrease in their values was observed and attributed to the crystallization process, lattice strain diminution, and cation redistribution