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排序方式: 共有161条查询结果,搜索用时 109 毫秒
1.
Andrzej Dabrowski Erich Robens Peter Klobes Klaus Meyer Przemyslaw Podkocielny 《Particle & Particle Systems Characterization》2003,20(5):311-322
Since a comprehensive survey published in 1999 [1] much work was done in standardizing measuring methods to characterize the surface geometry of dispersed and/or porous solids and to certify reference materials. The present paper is an extension of a short communication [2]. It gives a survey on existing standards and reports on new drafts and proposals. 相似文献
2.
Stanis aw Urban Maria Massalska-Arod
Albert Wü rflinger Roman D browski 《Liquid crystals》2003,30(3):313-318
The pressure-temperature ( p - T ) phase diagrams for four smectogenic members of the 4'-alkyl-4-cyanobiphenyl homologous series ( n CB, n =9, 10, 11 and 12) over the temperature range 320-410 K and pressure range 0.1-300 MPa (3 kbar) were constructed using DTA. At 1 atm 9CB exhibits nematic and smectic A d phases, while the other members show only the smectic A d phase. However, at elevated pressures the clearing line splits in the case of 10CB and 11CB which indicates the induction of a nematic phase. It was found that the triple point, where the isotropic, nematic and smectic phases coexist, is strongly shifted to higher pressures with increasing chain length. This was interpreted as being caused by a loss of the rod-like shape of the molecules containing longer alkyl tails which explore a range of conformations. The slope of the clearing line, d T /d p , depends strongly on the length of the alkyl chain for the n CB series, but does not show a step-wise change between the nematogenic and smectogenic members. 相似文献
3.
The basic principle of comparing the sample
mass with the mass of a reference body in equilibrium gives the equal-armed
beam balance a unique accuracy. Main parameters characterising the suitability
of the instrument are measuring range, resolution and relative sensitivity
(resolution/maximum load). The historical development of the values of these
parameters achieved depended strongly on the practical need in those times.
Technically unfavourable scales of the oldest Egyptian dynasties (~3000
BC) could resolve mass differences of 1 g and had a relative sensitivity of
at least 10–3. More sophisticated instruments
from the 18th Dynasty (~1567–1320 BC) achieved
a relative sensitivity of 10–4 independent
of the size of the instrument. In 350 BC Aristotle clarified the theory of
the lever and at about 250 BC Archimedes used the balance for density determinations
of solids. The masterpiece of a hydrological balance was Al Chazini’s
'Balance of Wisdom’ built about 1120. Its relative sensitivity
was 2⋅10–5.
Real progress took
place when scientists like Lavoisier (1743–1794) founded modern chemistry.
At the end of the 19th century metrological balances
reached a relative sensitivity of 10–9 with
a maximum load of several kilogrammes. That seems to be the high end of sensitivity
of the classical mechanical beam balance with knife edges. Improvements took
place by electrodynamic compensation (Emich, Gast).
In 1909 Ehrenhaft
and Millikan could weigh particles of 10–15
g by means of electrostatic suspension. In 1957 Sauerbrey invented the oscillating
quartz crystal balance. By observing the frequency shift of oscillating carbon
nanotubes or of silica nanorods, masses or mass changes in the attogram or
zeptogram have been observed recently. 相似文献
4.
E. Robens A. Dąbrowski V. V. Kutarov 《Journal of Thermal Analysis and Calorimetry》2004,76(2):647-657
Specific surface area and pore size distribution are determined usually from adsorption isotherms at low temperatures using
nitrogen or noble gases. These are not absolute parameters and the measuring methods are fraught with serious difficulties.
General problems of sorption measurements and recent developments are discussed. To obtain information for practical purposes
these measurements need to be supplemented by investigations of the sorbate/sorbent system used in practice. Results of the
measurement of nitrogen and water vapour adsorption on different materials are compared.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
5.
A need for an elaboration of the methods for synthesis and characterization of activated carbons with a requisite porous structure has existed for a long time. One of the methods giving possibility for creating controlled mesopore and micropore structures deals with the steam gasification of various carbon materials. In this work the effects of calcium catalyst on the catalytic steam gasification of active carbons from plum stones and porous polymers are presented. Determination of micropores capacity and specific adsorption in mesopores have been performed by means of the
s
method, but adsorption on the heterogeneous solids was described by the integral equation with various local isotherms. This equation has been solved by the regularization method. Based on this method the changes in structural parameters of active carbons depending on the amount of calcium catalyst were estimated.Nomenclature
d
width of slit-like micropore
-
F(x)
distribution function of the half-width
-
p
vapor pressure of sorbate
-
p/p
N
relative pressure
- PSAC
Plum Stone Active Carbon
-
average pore radius, nm
-
S
1
relative limit of the validity of experimental point on the adsorption isotherm in the computations by means of regularization method
- SAC
Synthetic Active Carbon
-
S
BET
specific surface area calculated by means of BET method, m2/g
-
S
mes
mesopore surface area, m2/g
-
S
mic
micropore surface area, m2/g
-
T
absolute temperature, K
-
V
mes
sorption capacity of mesopores, cm3/g
-
V
mic
sorption capacity of micropores, cm3/g
-
V
p
sorption capacity of pores, cm3/g
- w/w
weight in weight concentration
-
x
half-width of slit-like micropore, nm
-
x
1
maximum of half-width of micropore slit, nm
-
average half-width of slit-like micropore, nm
-
X
min-X
max
integration limits of thex
Greek Letters Greek Letters
variance of average half-width of slit-like micropore, nm2
-
local relative filling of micropores
-
total relative filling of micropores 相似文献
6.
Marek Da?browski 《Tetrahedron letters》2005,46(24):4175-4178
The metalation of (quasi)alkoxy-substituted dibromobenzenes C6H3(OR)Br2 with lithium diisopropylamide (LDA) has been investigated. For 1-(quasi)alkoxy-3,5-dibromobenzenes (R = Me, TMS), different selectivities were observed depending on reaction conditions and the size of the alkoxy group. The methoxy group was an effective ortho-director whereas this was not the case for the bulky trimethylsilyloxy group. The metalation of related 2,5-dibromoanisole was also examined showing a significant meta-directing effect by the methoxy group. The thermal stability of aryllithium intermediates is significantly lower when lithium is flanked by a bromine and a methoxy group, whereas 4-(quasi)alkoxy-2,6-dibromoaryllithiums are less labile. 相似文献
7.
A.?Spad?o E.?Otón R.?D?browski M.??urowska J.?M.?Otón N.?Bennis 《Opto-Electronics Review》2010,18(2):219-222
Electrooptical properties of several new orthoconic antiferroelectric liquid crystal mixtures with partially fluorinated alkoxyalkoxy
terminal chains have been investigated in order to select the best mixture for display applications. Electrooptical studies
have been performed on these orthoconic materials aiming at evaluating their static and dynamic performance under passive
multiplexing conditions. A number of parameters have been evaluated, static and dynamic contrast, driving scheme for passive
multiplexing, rise and fall response times, dynamic range, and dynamic greyscale. 相似文献
8.
R. Dǎbrowski J. Dziaduszek T. Szczuciński Z. Stolarzowa K. Czupryński 《Liquid crystals》2013,40(1):209-216
Abstract Bi-, tri- and tetra-ring liquid-crystalline compounds with an isothiocyanate group in the terminal position of the molecule and with an ethylene group in the central position have been synthesized. Phenyl, cyclohexyl, bicyclo[2,2,2]octyl, 1,3-dioxane and pyrimidinyl were used as the ring systems. Their transition temperatures and melting enthalpies have been determined. The relationship between the order of the rings in the molecule and the clearing points and melting points of the compounds is discussed. It is shown that these compounds are valuable components of liquid crystal mixtures, since they effectively raise their clearing points yet only have a small effect on the viscosity. 相似文献
9.
Glenn Mangelinckx Jeroen Beeckman Olga Chojnowska Jungsoon Shin James D.K. Kim Roman Dąbrowski 《Liquid crystals》2013,40(11):1553-1558
This paper describes a novel implementation of a dual-frequency liquid crystal optical shutter of the guest–host type. The transmissive state of the filter is obtained by applying a low-frequency electric field that brings the dichroic dye in a homeotropic orientation. The light-absorbing state is realised by a twisted planar configuration for which the absorption is quasi-independent of the polarisation. Switching between the two states occurs in about 1 ms and the devices show no scattering for wavelengths inside or outside the absorption band of the dichroic dye. Simulations and experiments reveal how a twisted state is obtained through the backflow phenomenon. 相似文献
10.
The results of dielectric studies performed in a broad frequency range for two compounds, 4-cyanophenyl 4-n-heptylbenzoate (7BBCN) and 4-(4-cyanobutyloxy)phenyl 4- n-heptylbenzoate (7BB4CN), are compared. They have the same molecular core whereas the strongly polar CN group is attached to the benzene ring in 7BBCN or is separated from it by the butyloxy chain in 7BB4CN. 7BBCN has a nematic phase, whereas 7BB4CN exhibits a monotropic nematic and smectic A2 polymorphism. Large differences in the dielectric properties of the two substances were found. The analysis of the results led to the conclusion that the antiparallel dipole-dipole associations are considerably stronger in the substance with a decoupled CN group. 相似文献