Search Results

You are looking at 11 - 20 of 625 items for :

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

Abstract  

The initial activation energy at zero conversionE o of thermooxidative decomposition has been taken as a measure of thermal stability of polycarbonates (PC) andE o has been correlated with the relative hydrolysis rater h as a measure of their hydrolyse resistance. It is suggested that both decomposition processes are initiated by the same mechanism, the attack of hydrolytic agent onto ester C−O bonds. The following values ofE o have been found: 187 (PC-M)>87 (PC-A)>43 (PC-C) kJ/mol, and they are correlated with values ofr h being 0.01 (PC-M)<1 (PC-A)<4.4 (PC-C). It has been found, using a computer modeling technique, that bothE o andr h depend on the minimized energy of conformations.

Restricted access

Abstract  

MDF cements using the blends of sulfoaluminate ferrite belite (SAFB) clinkers and ordinary Portland cement (OPC) in mass ratio 85:15 with Al2O3, and starch, polyphosphate (poly-P) or butylacrylate/acrylonitrile were subjected to moist atmospheres (ambient, 52 and 100% relative humidity (RH)) to investigate their moisture resistance. Their chemical, thermal, electron microscopic and magnetic properties were also studied before and after moisture attack. Butylacrylate/acrylonitrile (BA/AN) copolymer was found to be the most suitable for MDF cement synthesis since the sample containing BA/AN showed the best moisture resistant. There are significant differences in scanning electron microscopy (SEM) of MDF cements before and after moisture attack and with different polymers. New data on the paramagnetic nonhysteresis magnetization curves for all the samples are observed. The MDF cements synthesized from SAFB clinker with dissolved poly-P give the best signal/noise (S/N) ratio. Three main temperature regions on TG curves of both series of MDF cements are observed. In the inter-phase section of MDF cements, the content of classical cement hydrates decomposing by 250C is increased. Combustion of organic material took place by 550C. In the temperature range 550-800C, the decomposition of CaCO3 occurs.

Restricted access

The stability behaviour of a thin-film superconductor under a localized release of thermal disturbance is investigated. Two-dimensional conjugate film/substrate conduction equation with anisotropic thermal conductivity of the film, and Joule heat are employed to investigate effects of substrate and thermal properties on the intrinsic stability and quenching recovery. To consider the thermal boundary resistance between film and substrate, an interfacial-layer model (ILM) with very low diffusivity and an acoustic mismatch model (AMM) are employed. Results show that the thermal boundary resistance influences strongly the intrinsic stability. Thermal boundary resistance increases intrinsic stability if the thermal conductivity of the substrate or the disturbance energy is large. Higher Biot numbers and thermal conductivity ratios of film to substrate in longitudinal direction influence stability favorably. We demonstrate also that operation of a film/substrate system, such as YBCO/MgO, is either intrinsically stable or irrecoverably unstable.

Restricted access

Abstract  

The resistance against radiation of the tertiary pyridine resins synthesized for the treatment of spent nuclear fuels and high level radioactive waste was evaluated. After irradiation at 10 MGy, only approximately 10% or less of the exchange groups were lost in HCl solutions regardless of their concentrations, while 3040% were lost in HNO3. The pyridine resin has shown remarkable resistance against radiation particularly in HCl solution. It has been revealed that the decomposition of pyridine type resins results from the scission of the principal chains. An irradiation study was conducted also on the quaternary ammonium resins. Quatemization ratio was found to be reduced in HNO3 solutions at 10 MGy irradiation.

Restricted access

Abstract  

Sulphate resistance and passivation ability of the mortars made from pozzolan cement of CEM IV/A (P) type according to European Standard EN 197-1 (zeolite blended cement with 60.82 mass% of PC clinker, 35.09 mass% of zeolite and 4.09 mass% of gypsum abbreviated as ZBC) and ordinary Portland cement (abbreviated as PC) are introduced. Resistance tests were performed in water and 5% sodium sulphate solution (both 20°C) for 720 days. The increased sulphate resistance of pozzolan cement relative to that of PC was found. The key quantitative insight into the hydrate phase behaviour is given by thermal analysis. This is due to pozzolanic reaction of zeolite with PC resulting in reduction of the formed Ca(OH)2 opposite to the reference PC. Ability of pozzolan cements with 15 to 50 mass% of zeolite to protect steel against corrosion was verified in 20°C/85% RH-wet air within 180-day cure. Steel was not corroded in the mortars made with pozzolan cement containing up to 35 mass% of zeolite. Pozzolan cement of CEM IV/A (P) type containing 35 mass% of zeolite is a suitable cementitious material for concrete structures exposed to sulphate attack. Steel is protected against corrosion by this pozzolan cement in the same measure as the reference PC.

Restricted access

Abstract  

Gamma-induced intermediates stabilized in “kriptofix-21”, “kriptofix-22”, “kriptand-222” and 18-ane-S6 have been studied by ESR spectroscopy at 77–300 K. Apparently, radicals with H-atom abstraction from methylene group are the main paramagnetic products in diazocrown ethers while formation of both cyclic and uncyclic species is observed in 18-ane-S6 at 77 K. Heating γ-irradiated thiacrown ether up to room temperature mainly results in the cleavage of macrocyclic C−S following by stabilization of uncyclic radicals. 18-ane-S6 has a higher radiation resistance by comparison with oxygen- and aza-containing crown ethers at low temperature γ-radiolysis.

Restricted access

Abstract  

Fiber of Japanese food natto (Bacillus subtilis) is known to be superabsorbent poly(-glutamic acid) (PGA). NaCl particles precipitate in FeCl2-absorbed crosslinked PGA when heated at crystallization temperature of 320 °C for 10 to 60 min. After heat treatment the Mössbauer spectrum of FeCl2-crosslinked PGA consists of a quadrupole doublet due to FeCl2·2H2O. The Mössbauer spectrum of anhydrous FeCl2 reagent heated under the same condition shows an intense sextet due to -Fe2O3 . These results prove that the superabsorbent polymer, crosslinked PGA, has higher heat resistance.

Restricted access

Abstract  

Chitin and chitosan were irradiated in acidic solutions containing a decontaminating soap. It has been shown that these powders suspended in aqueous media do not undergo radiolysis, as their I.R. spectra are not altered, and there is no radiolysis product in the solution, except for the case of nitric acid at lowpH where some chitosan is present in dissolved form. These polymers therefore exhibit very good radiation resistance, in spite of the fact that the monomers undergo very rapid radiolysis.60Co radiations up to 50 000 krad were used. Under these conditions, chitosan does not lose its collection capacity for most of the metal ions of interest in the nuclear field, including cobalt, that can be collected from decontaminating soap solutions with a volume reduction of 10.

Restricted access

Abstract  

A phenomenological approach, in the parent phase of Cu-Zn-Al shape memory alloy, establishes a predictable model (or mathematical equations) relating the dependence of Ms with the temperature over a long period of time (i.e. seasonal or yearly room temperature). High-resolution resistance and temperature measurements vs. time are used. The long time Ms tracks the external room temperature via two temperature dependent time constants. In steady state, the changes in Ms approach17 per cent of the ‘room’ temperature change. The detailed analysis shows the puzzling disappearance of the after quench effects.

Restricted access

Abstract  

Gatifloxacin (GTN) was derivatized to its dithiocarbamate derivative and its radiolabeling with technetium-99m (99mTc) using the [99mTc≡N]2+ core was investigated. The appropriateness of the 99mTcN–gatifloxacin dithiocarbamate (99mTcN–GTND) complex as a potential multi-drug-resistance Streptococcus pneumoniae (MRSP) infection radiotracer was evaluated in terms of stability in saline, serum, in vitro binding with MRSP and biodistribution in artificially MRSP infected Male Wistar Rats (MWR). In saline the 99mTcN–GTND complex showed more than 90% labeling yield up to 4 h with a maximum yield of 98.25 ± 0.20%, after reconstitution. In serum the 99mTcN–GTND complex showed stability up to 16 h of incubation with the appearance of insignificant 15.95% undesirable side products. The 99mTcN–GTND complex demonstrated saturated in vitro binding with MRSP with a maximum value of 75.50 ± 1.00% (at 90 min). In MWR model of group A, almost six times higher uptake of the labeled GTND was monitored in the muscle of MWR infected with live MRSP as compared to the inflamed and normal muscles. Based on the higher labeling yield in saline, in vitro stability in serum, saturated in vitro binding with live MRSP and promising biodistribution in MWR model we recommend 99mTcN–gatifloxacin dithiocarbamate complex as a potential MRSP infection radiotracer.

Restricted access