共查询到10条相似文献,搜索用时 146 毫秒
1.
Thermal behaviour of the glass series (100-x)[50ZnO-10B2O3-40P2O5]·xSb2O3 (x=0-42 mol%) and (100-y)[60ZnO-10B2O3-30P2O5]·ySb2O3 (y=0-28 mol%) was investigated by DSC and TMA. The addition of Sb2O3 results in a decrease of the glass transition temperature and crystallization temperature in both compositional series. All
glasses crystallize on heating in the temperature range of 522–632°C. Thermal expansion coefficient of the glasses monotonously
increases with increasing Sb2O3 content in both series and varies within the range of 6.6–11.7 ppm °C−1. From changes of thermal capacity within the glass transition region it was concluded that with increasing Sb2O3 content the ‘fragility’ of the studied glasses increases. 相似文献
2.
A novel surfactant peptide consisting of an arginine cation with laurate anion has been synthesized, purified and characterized.
The critical micellar concentration (cmc) of peptide in aqueous solutions has been determined using spectroscopic techniques
and is found to increase from 0.06 to 0.11 mM with increasing temperature (15–45 °C). Cmc is also determined in the presence
of salts like NaCl, KCl and sodium acetate and it is found that these electrolytes hinder aggregation with a significant increase
in the case of sodium acetate. The aggregation number of the surfactant peptide has been determined using fluorescence quenching
measurements and is observed to decrease from 14 to 6 with increasing temperature (15–45 °C). The standard free energy change
(ΔG
0
m) and standard enthalpy change (ΔH
0
m) of the peptide aggregate are found to be negative with a small positive value for standard entropy change (ΔS
0
m). The peptide aggregate seems to undergo phase transition above 50 °C as observed from UV–vis and fluorescence spectroscopy.
From pyrene binding studies, it is shown that the interior dielectric constant increases from 5.08 at 34 °C to 8.77 at 50 °C
and further decreases with increase in temperature indicating a phase change at 50 °C. Also, the ratio of excimer intensity
to monomer intensity, which is a measure of microviscosity of the aggregate, decreases with increase in temperature with a
change at 50 °C indicating a phase change.
Received: 14 February 1997 Accepted: 13 August 1997 相似文献
3.
It has been shown that the swelling of poly-N-isopropyl-acrylamide (poly-NIPAM) microgel particles can be controlled by the addition of alcohols, in addition to the previously
observed effect of temperature. The degree of swelling is also controlled by the amount of cross-linker within the microgel
particles. At 25 °C, poly-NIPAM microgel particles collapse upon the addition of MeOH, EtOH and 2-PrOH to a minimum size and then, reswell again
as the alcohol-rich region is approached. This trend was also observed for poly-NIPAM microgel particles dispersed in 2-PrOH/water
mixtures upon heating to 50 °C. The particles, dispersed in either water or alcohol/water mixtures were found to be stable to flocculation between 25 °C and 50 °C.
Received: 27 February 1997 Accepted: 5 August 1997 相似文献
4.
P. Bermejo-Barrera Antonio Moreda-Piñeiro Jorge Moreda-Piñeiro Adela Bermejo-Barrera 《Analytical and bioanalytical chemistry》1997,358(7-8):844-847
Rapid methods were developed for the direct determination of Ag, Al, Cd and Mn in cocaine and heroin by ETAAS using programmes
omitting the charring step . Sample pretreatment was simple: dissolution in ultrapure water or in 35.0% (v/v) HNO3 for heroin or cocaine, respectively. Optimum drying temperatures were 250 °C for Ag, Al and Mn, and 300 °C for Cd. The run
cycles were 35 and 37 s, for Ag and Al respectively, and 36 s for Cd and Mn. The best results were obtained with Pd, Mg(NO3)2 and (NH4)2HPO4, as chemical modifiers. The limits of detection were 8.6, 55.9, 2.2 and 12.4 μg kg-1 for Ag, Al, Cd and Mn, respectively.
Received: 14 November 1996/Revised: 14 January 1997/Accepted: 18 January 1997 相似文献
5.
Uniform particles of manganese compounds obtained by forced hydrolysis of manganese(II) acetate 总被引:1,自引:0,他引:1
M. Ocaña 《Colloid and polymer science》2000,278(5):443-449
Procedures for the preparation at low temperature (80 °C) of uniform colloids consisting of Mn3O4 nanoparticles (about 20 nm) or elongated α-MnOOH particles with length less than 2 μm and width 0.4 μm or less, based on
the forced hydrolysis of aqueous manganese(II) acetate solutions in the absence (Mn3O4) or the presence (α-MnOOH) of HCl are described. These solids are only produced under a very restrictive range of reagent
concentrations involving solutions of 0.2–0.4 mol dm−3 manganese(II) acetate for Mn3O4 and of 1.6–2 mol dm−3 Mn(II) and 0.2–0.3 mol dm−3 HCl for α-MnOOH. The role that the acetate anions play in the precipitation of these solids is analyzed. It seems that these
anions promote the oxidation of Mn(II) to Mn(III), which readily hydrolyze causing precipitation. The evolution of the characteristics
of the powders with temperature up to 900 °C is also reported. Thus, Mn3O4 particles transform to Mn2O3 upon calcination at 800 °C; this is accompained by a sintering process. The α-MnOOH sample also experiences several phase
transformations on heating. First, it is oxidized at low temperatures (250–450 °C) giving MnO2 (pyrolusite), which is further reduced to Mn2O3 at 800 °C. After this process the particles still retain their elongated shape.
Received: 19 October 1999 Accepted: 24 November 1999 相似文献
6.
Silvana V. Rodrigues Decio Nepomuceno Luci Viana Martins W. Baumann 《Fresenius' Journal of Analytical Chemistry》1998,360(1):58-61
The solubility of two related series of each three triazine compounds has been determined in supercritical CO2, at 40 °C and pressures between 80 and 220⋅105 Pa. A high pressure small volume cell with adjustable optical path and built-in circulation pump has been constructed to
be used with quartz windows in a Perkin Elmer UV-VIS spectral photometer. At 220 bar solubilities are between 0.3 and 25 g/L,
dependent on the specific triazine structure. The isothermal pressure dependence of the solubility could be modelled using
the pressure dependent dielectric constant of CO2 as the only variable.
Received: 30 October 1996/Revised: 3 April 1997/Accepted: 13 April 1997 相似文献
7.
Sol and Gel state properties of aqueous gelatin solutions of concentrations 4%, 6%, 8% and 10% (w/v) have been investigated
through dielectric relaxation studies done at various temperatures in the range T=20–60 °C carried out over a frequency range f=20 Hz–10 MHz and no relaxation of any nature was observed. The sharp transition observed at the gelation temperature T
gel provided an excellent matching with the same measured through differential scanning calorimetry (DSC). The capacitance (C
p) values above f=100 kHz became increasingly negative as the gel was melted to the sol state. However, in the gel state C
p was found to be almost independent of temperature for frequencies above 100 kHz. At frequencies lower than 10 kHz, C
p measured was ∼105 F, implying pronounced interfacial polarization either due to electro-chemical reaction or because of ions getting trapped
at some interface within the bulk.
Received: 10 February 1997 Accepted: 2 September 1997 相似文献
8.
S. Lokhande S. A. Parab 《Russian Journal of Physical Chemistry A, Focus on Chemistry》2009,83(13):2346-2349
Determination of ion-exchange equilibrium constant (K) for Cl−/I− and Cl−/C2O42− system was studied at different temperatures from 25 to 45°C and by varying concentration of iodide and oxalate ion solution.
For both uni-univalent and uni-bivalent exchange systems, using 0.5 g of ion-exchange resin DUOLITE A-116 (in chloride form),
the value of K increases with rise in temperature i.e., from 13.0 at 25°C to 19.05 at 45°C for Cl−/I− system and 33.0 at 25°C to 63.0 at 45°C for Cl−/C2O42− system indicating the endothermic ion-exchange reaction. The difference in K values at the same temperature for the two was related to the ionic charge of exchangeable ions in the solution. 相似文献
9.
Wolfgang Preis Michael Holzinger Werner Sitte 《Monatshefte für Chemie / Chemical Monthly》2001,132(4):499-508
Summary. The van der Pauw method has been applied to conductivity relaxation experiments on YBa2Cu3O6+δ at 600°C in order to determine the chemical diffusion coefficient as a function of the oxygen partial pressure in the surrounding
atmosphere (100 > p
O
2/bar > 10−3). It is shown that the van der Pauw technique is suitable for monitoring the conductivity relaxation when the oxygen diffusion is perpendicular to the direct
current flowing through the sample in accordance with the van der Pauw geometry using thin tablets as samples. The oxygen partial pressure is changed stepwise (generally Δlogp
O
2 ≤ 0.5) by employing appropriate gas mixtures as well as an electrochemical oxygen pump device. An evaluation formula is given
for the determination of the chemical diffusion coefficient neglecting slow surface processes. In addition, the electronic
conductivity of YBa2Cu3O6+δ has been measured at 600°C as a function of oxygen partial pressure of the ambient atmosphere (100 > p
O
2/bar > 10−5) by means of the van der Pauw method applying the same experimental set-up. Typical values of the chemical diffusion coefficient are in the range of 10−6 cm2·s−1; the results of the conductivity measurements are interpreted in terms of an appropriate defect model.
Received May 30, 2000. Accepted June 8, 2000 相似文献
10.
Suwa K. Yamamoto K. Akashi M. Takano K. Tanaka N. Kunugi S. 《Colloid and polymer science》1998,276(6):529-533
We examined the effects of salt on the lower critical solution temperature (LCST) and lower critical solution pressure (LCSP)
of aqueous solutions of poly (N-vinylisobutyramide), polyNVIBA, and compared them with those on poly(N-isopropylacrylamide), polyNIPAAm. We found that the addition of salt (such as Na2SO4, NaCl, or KCl) decreased the LCST of aqueous polyNVIBA from 45 °C to below 20 °C, almost linearly with the salt concentrations
and dependent on the type of salt. We observed a similar concentration-dependent decrease in LCST for polyNIPAAm. When KI
or NaSCN was added to each aqueous polymer solution, some smaller increases in LCST were observed at relatively low salt concentrations;
higher concentrations of salt gave an almost linear decrease in LCST. As for LCSP, the addition of most types of salt lowered
the transition pressure, but the effects were much more dependent on the type and the valence of the salt (especially of anion)
in both polymers. Salt with divalent anion showed a larger decrease in LCSP, but those with mono valent anion showed a relatively
small decrease, even showed a slight increase at lower salt concentrations in the case of polyNVIBA. Salt with I- or SCN- showed evident increases in LCSP up to 1 M and was maintained higher than the control even at 2 M. We discuss the interactions
of the amide group in the side chains of polymers and water and their perturbation by ions.
Received: 13 November 1997 Accepted: 22 January 1998 相似文献