Search Results

You are looking at 131 - 140 of 627 items for :

  • "resistance" x
  • Chemistry and Chemical Engineering x
  • All content x
Clear All

Abstract  

The purpose of this work was to investigate the influence of titanium and yttrium dopants on chemical stability of selected Ba(Ce1−xTix)1−yYyO3 compounds. The presented results are the part of wider research concerning the crystallographic structure, microstructure, electrical and transport properties of these groups of materials. Samples of Ba(Ce1−xTix)1−yYyO3 with x=0.05, 0.07, 0.10, 0.15, 0.20, 0.30 and y=0.05, 0.10, 0.20 (for x=0.05) were prepared by solid-state reaction method. Initially, differential thermal analysis (DTA) and thermogravimetry (TG) were used for optimization of preparation conditions. Subsequently, DTA-TG-MS (mass spectrometry) techniques were applied for evaluation of the stability of prepared materials in the presence of CO2. The X-ray diffraction (XRD) and scanning electron microscopy (SEM) results were used to determine the phase composition, structure and microstructure of materials and to assist the interpretation of DTA-TG-MS results. The strong influence of Ti and Y dopants contents (x and y) on the properties was found. The introduction of Ti dopant led to the improvement of chemical stability against CO2. The lower Ti concentration the better resistance against CO2 corrosion was observed. Doping by Y had the opposite effect; the decrease of chemical stability was determined. In this case the higher Y dopant concentration the better resistance was observed. The attempt to correlate the influence of dopant on structure and chemical stability was also presented.

Restricted access

Improved inorganic ion-exchangers

II. Ammonium molybdophosphate—Silica gel system

Journal of Radioanalytical and Nuclear Chemistry
Authors: J. Doležal, J. Stejskal, M. Tympl, and V. Kouřím

Abstract  

The possibility of using the sol-gel method for preparation of inorganic ion-exchangers with a silica gel matrix has been demonstrated on the ammonium molybdophosphate-silica gel (AMP-SG) system. For the preparation of the ion-exchanger a sodium silicate solution, containing AMP and components to cause gelling to silica gel after increase of the temperature of the solution, is poured into a hot stirred silicone oil. The solution forms droplets, which are filtered off after their gelling, washed and dried. Beads containing 65 wt.% of AMP per gram of dry material have been prepared by this method and tested in ion-exchange columns for caesium removal from nitric acid solutions. Caesium may easily be desorbed with ammonium chloride or nitrate solutions. The ion-exchanger is suitable for long-time reversible column operation, having not only good chemical, thermal and radiation stabilities but also good mechanical and hydrodynamic properties and resistance to abrasion. It combines the advantage of the good kinetics of ion-exchange obtained with microparticles of precipitated inorganic ion-exchanger, with the low flow resistance of large particles.

Restricted access

Abstract  

Effect of hydrolysis time on molar mass, glass transition temperature, crystallinity, and resistance to thermooxidation at elevated temperatures was analyzed for Estanes 54600, 54610, and 54650. Kinetics of the hydrolysis can be plausibly described in terms of the first-order reaction with an average induction period of about 7 days. Reduction of molar mass induced by hydrolysis brings about an appreciable decrease in glass transition temperature, fraction of crystalline domains of soft segments, and thermooxidative stability. The latter effect is manifested by shortening of the lifetimes (related to 5% mass loss) the temperature dependence of which obeys the Arrhenius plot. The observed differences in hydrolysis resistance of Estanes can be related to their chemical composition.

Restricted access

Abstract  

Fire resistance of polyethylene is realized by magnesium hydroxide which is distinguished from halogenated fire-proofing agents by its lower cost and its non toxicity. Magnesium hydroxide decomposed by an endothermic reaction with liberation of water, contributing to fire proofing. The sample used (Kisuma 5A-N*) is constituted from a powder (0.6–0.8 micrometre) its surface is treated by plastic material in order to ameliorate its compatibility. We studied the thermal decomposition by DTA and TG, of mixtures constituted by polyethylene and magnesium hydroxide. A sudden decomposition began at 385°C for pure polyethylene and decomposition took place at 429°C for the mixture polyethylene-Kisuma (50–50). Incorporation of magnesium hydroxide in polyethylene increases fire resistance.

Restricted access

role between the catalyst and the reactants, the pores of the resin are so large that the reactants and the product are able to pass without any resistance. Fig. 2 Study on the internal mass transfer

Restricted access

. , 1981 . Effect of ageing on compression resistance and water stability of soil aggregates disturbed by tillage . Soil Tillage Res. 1 . 127 – 137 . Y ODER , R. E. , 1936

Restricted access

Abstract  

Cowpea (Vigna unguliculata Walp) is considered one of the most drought resistant species among the pulse crops. It was suggested that in the lower part of the stem, parenchymatous tissue for storing water has been developed for the function of drought resistance. However, such tissue has not been identified yet. In order to identify the water storing tissue in the stem of cowpea plant, the authors performed neutron radiography, which provides a non-destructive image of water distribution pattern in a plant. Common bean plant and soybean plant were used as references. Comparing the neutron radiograph for the stems of the plants, i.e., cowpea, common bean and soybean plants, the parenchymatous tissue with water storing function was distinguished in the intermode between primary leaf and the first trifoliate leaf specifically in cowpea plant.

Restricted access

Abstract  

We have studied the phenomenon of retention of uranium, as a carbonato complex UO2(CO3) 3 4− on titanium oxide. We first proposed a method for preparing titanium oxide whose ion-exchange capacities (2.9 meq Na+/g, 0.11 meq UO 2 2+ /g) and physical properties (mechanical resistance and granulometry) make it quite suitable for liquid chromatography and particularly for the extraction process of uranium from sea-water. We prepared the sodium tricarbonatouranate, the major form of uranium in sea-water. We studied the retention of this compound on titanium oxide. From a thermodynamic study of the retention equilibrium we proposed a retention mechanism of the ligang-exchange type: uranyl is retained on titanium oxide, surrounded with two CO 3 2− ligands and two ≥TiO ligands. We shown that 2/3 of the exchanged hydroxyl groups have pKH1=4.9 and 1/3 of them have pKH2=9.3.

Restricted access

Abstract  

The quality of a road relies on the good use of the on road surface and its maintenance along the years. The technology used and the corrections contributes to the prevention of early road destructions. Pavement with polmyer and others additives exhibits greater resistance to rutting, thermal cracking, and decreased fatigue damage, stripping and temperature susceptibility. Samples of CAP 40, SBS 440/02, L 1861/04, L 784/05 and L 2000/04 were analyzed by thermogravimetry (TG), derivative thermogravimetry (DTG), differential thermal analysis (DTA) and FTIR. Thermal characterizations showed that the main decomposition stage refers to asphaltenes and samples with additives exhibited a slight increase in thermal stability. The kinetic study, by Kissinger, showed that the sample with the highest stability was the SBS 440/02. Fourier transform infrared spectroscopy (FTIR) analysis suggested that these asphalt samples were originated from light oil.

Restricted access

Can one measure precise heat capacities with DSC or TMDSC?

A study of the baseline and heat-flow rate correction

Journal of Thermal Analysis and Calorimetry
Authors: J. Pak, W. Qiu, M. Pyda, E. Nowak-Pyda, and B. Wunderlich

Summary  

During a prior study of gel-spun fibers of ultrahigh-molar-mass polyethylene, a substantial error was observed on calculating the heat capacity with a deformed pan, caused by the lateral expansion of the fibers on shrinking during fusion. In this paper, the causes of this and other effects that limit the precision of heat capacity measurements by DSC and TMDSC are explored. It is shown that the major cause of error in the DSC is not a change in thermal resistance due to the limited contact of the fibers with the pan or the deformed pan with the platform, but a change in the baseline. In TMDSC, the frequency-dependence is changed. Since irreversible changes in the baseline can occur also for other reasons, inspections of the pan after the measurement are necessary for precision measurements.

Restricted access