53,578 research outputs found
Physical Properties of Traditional and Water-Miscible Oil Paints as Assessed By Single-Sided Nmr
Single-sided NMR has been demonstrated as a useful technique for the inexpensive and non-invasive study of cultural heritage objects, including numerous different painting and paint samples. The relatively recent invention of water-miscible oil paints – a new form of environmentally friendly oil paint that can be thinned and cleaned by water – provides a need for analysis of the physical properties of the cured paint films. Single-sided NMR offers an excellent analytical tool to study the structural effects of the emulsifying agent present in water-miscible oil paints on the paint linoxyn network by measuring the transverse (T2) relaxation times for various pigments. In this research, single-sided NMR is shown to be a successful technique in analyzing the physical properties of oil paint networks in comparison to the chemical composition of the paints as assessed by fatty acid ratios produced from complimentary GCMS data. The discovery of a correlation between specific fatty acid ratios and relaxation times suggests that the presence of the emulsifying agent interferes with autoxidation and the cross-linking of the paint network, impeding the relative rate of curing. The conclusions drawn from this research offer the potential for multiple new experiments to better understand the chemistry behind the curing of oil paints in the presence of an emulsifying agent
Colossal dielectric constants in transition-metal oxides
Many transition-metal oxides show very large ("colossal") magnitudes of the
dielectric constant and thus have immense potential for applications in modern
microelectronics and for the development of new capacitance-based
energy-storage devices. In the present work, we thoroughly discuss the
mechanisms that can lead to colossal values of the dielectric constant,
especially emphasising effects generated by external and internal interfaces,
including electronic phase separation. In addition, we provide a detailed
overview and discussion of the dielectric properties of CaCu3Ti4O12 and related
systems, which is today's most investigated material with colossal dielectric
constant. Also a variety of further transition-metal oxides with large
dielectric constants are treated in detail, among them the system La2-xSrxNiO4
where electronic phase separation may play a role in the generation of a
colossal dielectric constant.Comment: 31 pages, 18 figures, submitted to Eur. Phys. J. for publication in
the Special Topics volume "Cooperative Phenomena in Solids: Metal-Insulator
Transitions and Ordering of Microscopic Degrees of Freedom
Prediction of long and short time rheological behavior in soft glassy materials
We present an effective time approach to predict long and short time
rheological behavior of soft glassy materials from experiments carried out over
practical time scales. Effective time approach takes advantage of relaxation
time dependence on aging time that allows time-aging time superposition even
when aging occurs over the experimental timescales. Interestingly experiments
on variety of soft materials demonstrate that the effective time approach
successfully predicts superposition for diverse aging regimes ranging from
sub-aging to hyper-aging behaviors. This approach can also be used to predict
behavior of any response function in molecular as well as spin glasses.Comment: 13 pages, 4 figure
Effects of zinc oxide filler on the curing and mechanical response of alkyd coatings
The mechanical properties of an alkyd resin filled with zinc oxide pigment were studied at different concentrations over a wide range of time scales using dynamic mechanical analysis, quartz crystal rheometry and nanoindentation. The motivation for this work stems from the interest in accessing the long-term properties of paint coatings by studying the mechanical properties of historic paints. In this foundational work, we compare three different modalities of mechanical measurements and systematically determine the effect of pigment filler loading on the measured properties. Quantitative agreement between the methods is obtained when the characteristic time scales of each of the methods is taken into account. While nanoindentation is the technique most readily applied to historic paint samples, the rheometric quartz crystal microbalance (rheo-QCM) is the best suited for obtaining mechanistic information from measurements of paint properties over time, provided that appropriate thin-film samples can be produced. In these studies we find that ZnO increases the rate of oxidation of the alkyd during the initial stages of cure by an amount that depends on the ZnO content
Non-intrinsic origin of the Colossal Dielectric Constants in CaCu3Ti4O12
The dielectric properties of CaCu3Ti4O12, a material showing colossal values
of the dielectric constant, were investigated in a broad temperature and
frequency range extending up to 1.3 GHz. A detailed equivalent circuit analysis
of the results and two crucial experiments, employing different types of
contacts and varying sample thickness, provide clear evidence that the
apparently high values of the dielectric constant in CaCu3Ti4O12 are
non-intrinsic and due to electrode polarization effects. The intrinsic
properties of CaCu3Ti4O12 are characterized by charge transport via hopping of
localized charge carriers and a relatively high dielectric constant of the
order of 100.Comment: 4 pages, 4 figure
Dielectric properties of Li2O-3B2O3 glasses
The frequency and temperature dependence of the dielectric constant and the
electrical conductivity of the transparent glasses in the composition
Li2O-3B2O3 (LBO) were investigated in the 100 Hz- 10 MHz frequency range. The
dielectric constant and the loss in the low frequency regime were electrode
material dependent. Dielectric and electrical relaxations were respectively
analyzed using the Cole-Cole and electric modulus formalisms. The dielectric
relaxation mechanism was discussed in the framework of electrode and charge
carrier (hopping of the ions) related polarization using generalized Cole-Cole
expression. The frequency dependent electrical conductivity was rationalized
using Jonscher's power law. The activation energy associated with the dc
conductivity was 0.80 \pm 0.02 eV, which was ascribed to the motion of Li+ ions
in the glass matrix. The activation energy associated with dielectric
relaxation was almost equal to that of the dc conductivity, indicating that the
same species took part in both the processes. Temperature dependent behavior of
the frequency exponent (n) suggested that the correlated barrier hopping model
was the most apposite to rationalize the electrical transport phenomenon in
Li2O-3B2O3 glasses. These glasses on heating at 933 K/10h resulted in the known
non-linear optical phase LiB3O5.Comment: 32 pages, 13 figure
Apparent giant dielectric constants, dielectric relaxation, and ac-conductivity of hexagonal perovskites La1.2Sr2.7BO7.33 (B = Ru, Ir)
We present a thorough dielectric investigation of the hexagonal perovskites
La1.2Sr2.7IrO7.33 and La1.2Sr2.7RuO7.33 in a broad frequency and temperature
range, supplemented by additional infrared measurements. The occurrence of
giant dielectric constants up to 10^5 is revealed to be due to electrode
polarization. Aside of dc and ac conductivity contributions, we detect two
intrinsic relaxation processes that can be ascribed to ionic hopping between
different off-center positions. In both materials we find evidence for charge
transport via hopping of localized charge carriers. In the infrared region,
three phonon bands are detected, followed by several electronic excitations. In
addition, these materials provide further examples for the occurrence of a
superlinear power law in the broadband ac conductivity, which recently was
proposed to be a universal feature of all disordered matter.Comment: 8 pages, 7 figure
Multiferroic behavior in CdCr2X4 (X = S, Se)
The recently discovered multiferroic material CdCr2S4 shows a coexistence of
ferromagnetism and relaxor ferroelectricity together with a colossal
magnetocapacitive effect. The complex dielectric permittivity of this compound
and of the structurally related CdCr2Se4 was studied by means of broadband
dielectric spectroscopy using different electrode materials. The observed
magnetocapacitive coupling at the magnetic transition is driven by enormous
changes of the relaxation dynamics induced by the development of magnetic
order
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