828 research outputs found
Role of carbonates in the chemical evolution of sodium carbonate-activated slag binders
Multi-technique characterisation of sodium carbonate-activated blast furnace slag binders was conducted in order to determine the influence of the carbonate groups on the structural and chemical evolution of these materials. At early age (<4 days) there is a preferential reaction of Ca2+ with the CO3 2â from the activator, forming calcium carbonates and gaylussite, while the aluminosilicate component of the slag reacts separately with the sodium from the activator to form zeolite NaA. These phases do not give the high degree of cohesion necessary for development of high early mechanical strength, and the reaction is relatively gradual due to the slow dissolution of the slag under the moderate pH conditions introduced by the Na2CO3 as activator. Once the CO3 2â is exhausted, the activation reaction proceeds in similar way to an NaOH-activated slag binder, forming the typical binder phases calcium aluminium silicate hydrate and hydrotalcite, along with Ca-heulandite as a further (Ca,Al)-rich product. This is consistent with the significant gain in compressive strength and reduced porosity observed after 3 days of curing. The high mechanical strength and reduced permeability developed in these materials beyond 4 days of curing elucidate that Na2CO3-activated slag can develop desirable properties for use as a building material, although the slow early strength development is likely to be an issue in some applications. These results suggest that the inclusion of additions which could control the preferential consumption of Ca2+ by the CO3 2â might accelerate the reaction kinetics of Na2CO3-activated slag at early times of curing, enhancing the use of these materials in engineering applications
Universal Holographic Chiral Dynamics in an External Magnetic Field
In this work we further extend the investigation of holographic gauge
theories in external magnetic fields, continuing earlier work. We study the
phenomenon of magnetic catalysis of mass generation in 1+3 and 1+2 dimensions,
using D3/D7- and D3/D5-brane systems, respectively. We obtain the low energy
effective actions of the corresponding pseudo Goldstone bosons and study their
dispersion relations. The D3/D7 system exhibits the usual
Gell-Mann--Oakes--Renner (GMOR) relation and a relativistic dispersion
relation, while the D3/D5 system exhibits a quadratic non-relativistic
dispersion relation and a modified linear GMOR relation. The low energy
effective action of the D3/D5 system is related to that describing magnon
excitations in a ferromagnet. We also study properties of general Dp/Dq systems
in an external magnetic field and verify the universality of the magnetic
catalysis of dynamical symmetry breaking.Comment: 41 pages, 11 figures, references adde
Precise measurements of radio-frequency magnetic susceptibility in (anti)ferromagnetic materials
Dynamic magnetic susceptibility, , was studied in several intermetallic
materials exhibiting ferromagnetic, antiferromagnetic and metamagnetic
transitions. Precise measurements by using a 14 MHz tunnel diode oscillator
(TDO) allow detailed insight into the field and temperature dependence of
. In particular, local moment ferromagnets show a sharp peak in
near the Curie temperature, . The peak amplitude decreases and shifts to
higher temperatures with very small applied dc fields. Anisotropic measurements
of CeVSb show that this peak is present provided the magnetic easy axis is
aligned with the excitation field. In a striking contrast, small moment,
itinerant ferromagnets (i.e., ZrZn) show a broad maximum in that
responds differently to applied field. We believe that TDO measurements provide
a very sensitive way to distinguish between local and itinerant moment magnetic
orders. Local moment antiferromagnets do not show a peak at the N\'eel
temperature, , but only a sharp decrease of below due to the
loss of spin-disorder scattering changing the penetration depth of the ac
excitation field. Furthermore, we show that the TDO is capable of detecting
changes in spin order as well as metamagnetic transitions. Finally, critical
scaling of in the vicinity of is discussed in CeVSb and
CeAgSb
Spin diffusion at finite electric and magnetic fields
Spin transport properties at finite electric and magnetic fields are studied
by using the generalized semiclassical Boltzmann equation. It is found that the
spin diffusion equation for non-equilibrium spin density and spin currents
involves a number of length scales that explicitly depend on the electric and
magnetic fields. The set of macroscopic equations can be used to address a
broad range of the spin transport problems in magnetic multilayers as well as
in semiconductor heterostructure. A specific example of spin injection into
semiconductors at arbitrary electric and magnetic fields is illustrated
Nanostructural characterization of geopolymers by advanced beamline techniques
This paper presents the outcomes of a series of beamline-based studies, the results of which are combined to provide a more detailed multiscale understanding of the structure and chemistry of geopolymer binders.
The range of beamline-based characterization techniques which have been applied to the study of geopolymer binders is increasing rapidly; although no single technique can provide a holistic view of binder structure across all the length scales which are of importance in determining strength development and durability, the synergy achievable through the combination of multiple beamline techniques is leading to rapid advances in knowledge in this area. Studies based around beamline infrared and X-ray fluorescence microscopy, in situ and ex situ neutron pair distribution function analysis, and nano- and micro-tomography, are combined to provide an understanding of geopolymer gel chemistry, nano- and microstructure in two and three dimensions, and the influences of seeded nucleation and precursor chemistry in these key areas.
The application of advanced characterization methods in recent years has brought the understanding of geopolymer chemistry from a point, not more than a decade ago, when the analysis of the detailed chemistry of the aluminosilicate binder gel was considered intractable due to its disordered (âX-ray amorphousâ) nature, to the present day where the influence of key compositional parameters on nanostructure is well understood, and both gel structure and reaction kinetics can be manipulated through methods including seeding, temperature variation, and careful mix design.
This paper therefore provides a review outlining the value of nanotechnology â and particularly nanostructural characterization â in the development and optimization of a new class of environmentally beneficial cements and concretes. Key engineering parameters, in particularly strength development and permeability, are determined at a nanostructural level, and so it is essential that gel structures can be analyzed and manipulated at this level; beamline-based characterization techniques are critical in providing the ability to achieve this goal
Operator product expansion of higher rank Wilson loops from D-branes and matrix models
In this paper we study correlation functions of circular Wilson loops in
higher dimensional representations with chiral primary operators of N=4 super
Yang-Mills theory. This is done using the recently established relation between
higher rank Wilson loops in gauge theory and D-branes with electric fluxes in
supergravity. We verify our results with a matrix model computation, finding
perfect agreement in both the symmetric and the antisymmetric case.Comment: 28 pages, latex; v2: minor misprints corrected, references adde
Holographic Spectral Functions in Metallic AdS/CFT
We study the holographic D3/D7 setup dual to N=4 supersymmetric Yang-Mills
with quenched fundamental matter. We extend the previous analyses of
conductivity and photoproduction to the case where there is a finite electric
field. Due to the electric field a special region in the D7-brane geometry,
labelled the singular shell, appears generically, and the computation of
correlators involves a careful study of the indicial exponents both at this
singular region and at the horizon. We show that there is a unique choice
consistent with the known expression for the electrical conductivity found by
Karch and O'Bannon. We explore the parameter space spanned by the quark mass,
the baryon density and the electric field. We find a region where the
conductivity and photoproduction change rapidly and trace this behavior to
competing effects which manifest themselves as a crossover behavior in the
probe brane embeddings.Comment: 30 pages, 13 figures, v2. references added, minor corrections mad
On unquenched N=2 holographic flavor
The addition of fundamental degrees of freedom to a theory which is dual (at
low energies) to N=2 SYM in 1+3 dimensions is studied. The gauge theory lives
on a stack of Nc D5 branes wrapping an S^2 with the appropriate twist, while
the fundamental hypermultiplets are introduced by adding a different set of Nf
D5-branes. In a simple case, a system of first order equations taking into
account the backreaction of the flavor branes is derived (Nf/Nc is kept of
order 1). From it, the modification of the holomorphic coupling is computed
explicitly. Mesonic excitations are also discussed.Comment: 25 pages, 4 figure
Time-Dependent Spintronic Transport and Current-Induced Spin Transfer Torque in Magnetic Tunnel Junctions
The responses of the electrical current and the current-induced spin transfer
torque (CISTT) to an ac bias in addition to a dc bias in a magnetic tunnel
junction are investigated by means of the time-dependent nonquilibrium Green
function technique. The time-averaged current (time-averaged CISTT) is
formulated in the form of a summation of dc current (dc CISTT) multiplied by
products of Bessel functions with the energy levels shifted by . The tunneling current can be viewed as to happen between the photonic
sidebands of the two ferromagnets. The electrons can pass through the barrier
easily under high frequencies but difficultly under low frequencies. The tunnel
magnetoresistance almost does not vary with an ac field. It is found that the
spin transfer torque, still being proportional to the electrical current under
an ac bias, can be changed by varying frequency. Low frequencies could yield a
rapid decrease of the spin transfer torque, while a large ac signal leads to
both decrease of the electrical current and the spin torque. If only an ac bias
is present, the spin transfer torque is sharply enhanced at the particular
amplitude and frequency of the ac bias. A nearly linear relation between such
an amplitude and frequency is observed.Comment: 13 pages,8 figure
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