Electrical properties of ZnO varistors prepared by direct mixing of constituent phases

Varistor samples containing different amounts of constituent phases were prepared by direct mixing of constituent phases. Detailed electrical characterization was performed to explain the influence of minor phases (spinel and intergranular phases) on overall properties. Characterization included investigation of the non-linear coefficients (α), breakdown electric field (EB), leakage currents (JL), grain boundary barrier hight (ΦB) and constant β from current-voltage characteristics, as well as calculation of activation energies for conduction (EA) from ac impedance spectroscopy in the temperature interval 30-410°C. Varistors sintered at 1100 °C for 1 h showed pronounced differences in electrical properties depending on relative molar ratios of the phases. Results were discussed in the sense of possible reduction of the content of minor phases in ZnO varistors

Electrical properties of mechanically activated zinc oxide

Microstructural properties of a commercial zinc oxide powder were modified by mechanical activation in a high-energy vibro-mill. The obtained powders were dry pressed and sintered at 1100°C for 2 h. The electrical properties of grain boundaries of obtained ZnO ceramics were studied using an ac impedance analyzer. For that purpose, the ac electrical response was measured in the temperature range from 23 to 240°C in order to determine the resistance and capacitance of grain boundaries. The activation energies of conduction were obtained using an Arrhenius equation. Donor densities were calculated from Mott-Schottky measurements. The influence of microstructure, types and concentrations of defects on electrical properties was discussed

Factors affecting indoor radon variations: A case study in schools of eastern macedonia

The subject of this study is the radon concentrations variations, measured with a nuclear track detectors in a total of 58 premises in all 29 primary schools of 4 municipalities in the Eastern part of the Republic of Macedonia. Despite a relatively small territory, the variability of radon concentrations proved to be significant. The geometric means (geometric standard deviations) of radon concentrations in the examined municipalities were in the range from GM = 71 Bq/m 3 (GSD = 2.08) to GM = 162 Bq/m 3 (GSD = 2.69), while for the entire region it was: GM = 96 Bq/m 3 (GSD = 2.47). The influence of the geographical and geological features of the school site as well as the building characteristics on the radon variations were investigated. The analysis showed that type of municipality, building materials, basement and geology have significant effects and respectively describe 6%, 16%, 22%, 39% of the radon total variability

Chemical Stability of Doped BaCe0.9Y0.1O3-x as a Proton Conducting Electrolyte for IT-SOFC

BaCe0.9Y0.1O3-δ (BCY) has been one of the most studied materials known for its highest proton conductivity at temperatures between 500 and 700 ºC, which allows its application as a proton conducting electrolyte for intermediate-temperature solid oxide fuel cells (IT-SOFC). The proton conductivity is an exclusive property of mixed oxides with perovskite structure and large unit cell volume, such as BaCeO3 or SrCeO3. Doping with aliovalent cations (Y3+) that replace Ce4+ induces formation of point defects (oxygen vacancies), which in wet or hydrogen containing atmosphere allow proton mobility. The main disadvantage of this material is its instability in CO2-rich atmosphere due to the basic character of the crystal lattice, thus limiting its application in SOFCs in respect to fuel selection. However, the stability of BCY can be enhanced by doping with cations that may raise the acidic character of the material, such as Nb5+, Ta5+ or In3+. Introduction of pentivalent cations will lead to reduced amount of point defects and consequently lower proton conductivity and it is therefore recommended that their molar concentration should not exceed 5 %. On the other hand, trivalent In3+ is more suitable as it can completely replace Y3+ since it can both serve as a point defect source and increase acidity of the crystal lattice. Because of these properties it can be introduced in much larger amounts than Nb5+ or Ta5+. In this study BaCe0.9-xNbxY0.1O3-δ (where x = 0.01, 0.03 and 0.05) and BaCe1-xInxO3-δ (where x = 0.15, 0.20 and 0.25) powders were synthesized by the method of autocombustion, while BaCe0.9-xTaxY0.1O3-δ (where x = 0.01, 0.03 and 0.05) powders were prepared by the classical solid state route. Much higher specific surface areas were observed for the samples synthesized by the autocombustion method. In the case of Nb and Ta doped samples, the dense electrolytes were formed after sintering at 1550 ºC for 5 h in air. Temperature of 1300 ºC was enough to complete sintering of the samples doped with In after 5 h in air, which was another advantage of In as a dopant. The conductivities determined by impedance measurements in temperature range of 550-700 ºC in wet hydrogen showed a decreasing trend with increase of Nb and Ta content, while it was the opposite in the case of In. Interestingly, the total conductivity of the samples BaCe0.85Nb0.05Y0.1O3-δ, BaCe0.85Ta0.05Y0.1O3-δ and BaCe0.75In0.25O3-δ reached around 5×10–3 S/cm in wet hydrogen atmosphere at 700 ºC. After exposure in 100 % CO2 atmosphere at 700 ºC for 5 h, the samples were investigated by X-ray analysis. It was found that even 15 % In could completely supress degradation of electrolyte, while the highest concentrations of Nb and Ta (5%) were necessary to secure sufficient stability in CO2

High annual radon concentration in dwellings and natural radioactivity content in nearby soil in some rural areas of Kosovo and Metohija

Some previous studies on radon concentration in dwellings of some areas of Kosovo and Metohija have revealed a high average radon concentration, even though the detectors were exposed for three months only. In order to better design a larger study in this region, the annual measurements in 25 houses were carried out as a pilot study. For each house, CR-39-based passive devices were exposed in two rooms for the two consecutive six-month periods to account for seasonal variations of radon concentration. Furthermore, in order to correlate the indoor radon with radium in nearby soil and to improve the knowledge of the natural radioactivity in the region, soil samples near each house were collected and 226Ra, 232Th, 40K activity concentration were measured. The indoor radon concentration resulted quite high from the average (163 Bq/m3) and generally it did not differ considerably between the two rooms and the two six-month periods. The natural radionuclides in soil resulted to be distributed quite uniformly. Moreover, the correlation between the226Ra content in soil and radon concentration in dwellings resulted to be low (R2=0.26). The annual effective dose from radon and its short-lived progeny (5.5 mSv, in average) was calculated by using the last ICRP dose conversion factors. In comparison, the contribution to the annual effective dose of outdoor gamma exposure from natural radionuclides in soil is nearly negligible (66 mSv). In conclusion, the observed high radon levels are only partially correlated with radium in soil; moreover, a good estimate of the annual average of radon concentration can be obtained from a six-month measurement with a proper choice of exposure period, which could be useful when designing large surveys

Textural properties of macroporous acid modified montmorillonite nanocomposites

Macroporous crosslinked copolymer, poly(glycidyl methacrylate-co-ethylene glycol dimethacrylate) and its nanocomposites with acid modified montmorillonite (WA) were synthesized by radical suspension copolymerization. Nanocomposites were obtained by introducing various amounts of WA into the reaction system. Textural properties of synthesized samples were analyzed by mercury intrusion porosimetry. The synthesized nanocomposites have significantly higher specific surface area in comparison to the copolymer. Total pore volume and the most dominant pore diameter decrease with incorporation of acid modified montmorillonite in copolymer matrix.Physical chemistry 2012 : 11th international conference on fundamental and applied aspects of physical chemistry; Belgrade (Serbia); 24-28 September 201

Modifikovani bentonit kao adsorbent i katalizator za prečišćavanje otpadnih voda koje sadrže boje

Modification and characterization of bentonite from location Bogovina, Serbia was performed in order to obtain material applicable in wastewater purification. The LT 75μm bentonite fraction was used in organobentonite synthesis while the LT 2μm bentonite fraction, obtained by hydroseparation was used in pillaring procedure. Organo-modification of bentonite was performed with (1-hexadecyl)trimethylammonium bromide (HDTMA-Br). Pillared bentonite was obtained using standard procedure. Al3+ and Fe3+ were incorporated in pillars in 4:1 ratio and applied as catalyst in catalytic wet peroxide oxidation. Differences in structure of starting and modified bentonites were established by XRD analysis and nitrogen physisorption on -196°C. The (001) smectite peak around 2θ = 6° shifts during the modification process. The Na-exchange process lowered d001 from 1.53 nm (2θ = 5.78°) for starting clay to 1.28 nm (2θ = 6.92°), but the clay retained its swelling properties. The pillaring process increased and fixed the basal spacing to 1.74 nm. Intercalation of HDTMA ions into smectite structure increased d001 to 2.00 nm for organobentonite. Specific surface area, SBET, was affected by particle size and type of modification. The samples with finer bentonite fraction had higher SBET due to increased smectite content. Na-exchanged bentonite samples had higher SBET value than starting clay samples of same granulation. Organo modification caused dramatic decrease in SBET value, while the pillaring process lead to an increase of SBET value. Adsorptive and catalytic purification of wastewaters containing dyes was tested using Acid Yellow 99 as a model dye. Na-exchanged bentonite had greater adsorption affinity for dye adsorption than raw bentonite owing to higher SBET. By organomodification this affinity was enhanced more than 70 times due to transition of bentonite surface from hydrophilic to organophilic. Al, Fe pillared bentonite was proven to be efficient in catalytic wet peroxide oxidation of Acid Yellow 99 dye at room temperature.Izvršena je modifikacija i karakterizacija bentonita radi dobijanja materijala za prečišćavanje otpadnih voda. Organo-modifikacija bentonita izvedena je pomoću heksadeciltrimetilamonijum-bromida. Pilareni bentonit dobijen je standardnim postupkom uz ugradnju Al3+ i Fe3+ jona u odnosu 4:1, i korišćen je kao katalizator za oksidativnu razgradnju u prisustvu vodonik-peroksida. Razlike u strukturi polaznog i modifikovanih bentonita utvrđene su rendgeno-strukturnom analizom i fizisorpcijom azota na -196°C. Ispitivano je adsorpciono i katalitičko prečišćavanje otpadnih voda koje sadrže boje, korišćenjem Acid Yellow 99 kao model boje. Izvršeno je poređenje katalitičkih i adsorpiconih svojstava modifikovanih bentonita

Sorption of different phenol derivatives on a functionalized macroporous nanocomposite of poly(glycidyl methacrylate-co-ethylene glycol dimethacrylate) and acid modified bentonite

Macroporous nanocomposite of poly(glycidyl methacrylate-co-ethylene glycol dimethacrylate) and acid modified bentonite was prepared by radical suspension copolymerization. Nanocomposite was functionalized with diethylenetriamine (DETA), by ring-opening reaction of the pendant epoxy groups. Functionalization was performed in order to enable phenol derivatives sorption. This new, not sufficiently investigated material, with developed porous structure was denoted CP-S-A-DETA. In this study, the influence of temperature on 4-nitrophenol (4NP) sorption on CP-S-A-DETA was investigated. The chemisorption was estimated as dominant process since activation energy of sorption of 4NP of 54.8 kJ mol(-1) was obtained. After determining the optimal sorption conditions for 4NP, the sorption of 2-nitrophenol (2NP) and 2-chloro-4-nitrophenol (2Cl4NP) on CP-S-A-DETA was investigated with respect to pH, initial concentration and contact time. The 2NP sorption was seldom tested, while according to our knowledge, the 2Cl4NP sorption was not investigated. The isotherm data were best fitted with Langmuir model, while the sorption dynamics obeyed the pseudo-second-order kinetic model for all derivatives

Factors affecting indoor radon variations: A case study in schools of eastern macedonia

The subject of this study is the radon concentrations variations, measured with a nuclear track detectors in a total of 58 premises in all 29 primary schools of 4 municipalities in the Eastern part of the Republic of Macedonia. Despite a relatively small territory, the variability of radon concentrations proved to be significant. The geometric means (geometric standard deviations) of radon concentrations in the examined municipalities were in the range from GM = 71 Bq/m 3 (GSD = 2.08) to GM = 162 Bq/m 3 (GSD = 2.69), while for the entire region it was: GM = 96 Bq/m 3 (GSD = 2.47). The influence of the geographical and geological features of the school site as well as the building characteristics on the radon variations were investigated. The analysis showed that type of municipality, building materials, basement and geology have significant effects and respectively describe 6%, 16%, 22%, 39% of the radon total variability