3,090 research outputs found
Material characterization of structural adhesives in the lap shear mode
A general method for characterizing structual adhesives in the bonded lap shear mode is proposed. Two approaches in the form of semiempirical and theoretical approaches are used. The semiempirical approach includes Ludwik's and Zhurkov's equations to describe respectively, the failure stresses in the constant strain rate and constant stress loading modes with the inclusion of the temperature effects. The theoretical approach is used to describe adhesive shear stress-strain behavior with the use of viscoelastic or nonlinear elastic constitutive equations. Two different model adhesives are used in the single lap shear mode with titanium adherends. These adhesives (one of which was developed at NASA Langley Research Center) are currently considered by NASA for possible aerospace applications. Use of different model adhesives helps in assessment of the generality of the method
Energy decay for the damped wave equation under a pressure condition
We establish the presence of a spectral gap near the real axis for the damped
wave equation on a manifold with negative curvature. This results holds under a
dynamical condition expressed by the negativity of a topological pressure with
respect to the geodesic flow. As an application, we show an exponential decay
of the energy for all initial data sufficiently regular. This decay is governed
by the imaginary part of a finite number of eigenvalues close to the real axis.Comment: 32 page
Artificial decellularized extracellular matrix improves the regenerative capacity of adipose tissue derived stem cells on 3D printed polycaprolactone scaffolds
Ideal tissue engineering frameworks should be both an optimal biological microenvironment and a shape and stability providing framework. In this study we tried to combine the advantages of cell-derived artificial extracellular matrix (ECM) with those of 3D printed polycaprolactone (PCL) scaffolds. In Part A, both chondrogenic and osteogenic ECMs were produced by human adipose derived stem cells (hASCs) on 3D-printed PCL scaffolds and then decellularized to create cell free functionalized PCL scaffolds, named acPCL and aoPCL respectively. The decellularization resulted in a significant reduction of the DNA content as well as the removal of nuclei while the ECM was largely preserved. In Part B the bioactivation and the effect of the ac/aoPCL scaffolds on the proliferation, differentiation, and gene expression of hASCs was investigated. The ac/aoPCL scaffolds were found to be non-toxic and allow good adhesion, but do not affect proliferation. In the in vitro investigation of cartilage regeneration, biochemical analysis showed that acPCL scaffolds have an additional effect on chondrogenic differentiation as gene expression analysis showed markers of cartilage hypertrophy. The aoPCL showed a large influence on the differentiation of hASCs. In control medium they were able to stimulate hASCs to produce calcium alone and all genes relevant investigated for osteogenesis were significantly higher expressed on aoPCL than on unmodified PCL. Therefore, we believe that ac/aoPCL scaffolds have a high potential to improve regenerative capacity of unmodified PCL scaffolds and should be further investigated
Hole doping dependences of the magnetic penetration depth and vortex core size in YBa2Cu3Oy: Evidence for stripe correlations near 1/8 hole doping
We report on muon spin rotation measurements of the internal magnetic field
distribution n(B) in the vortex solid phase of YBa2Cu3Oy (YBCO) single
crystals, from which we have simultaneously determined the hole doping
dependences of the in-plane Ginzburg-Landau (GL) length scales in the
underdoped regime. We find that Tc has a sublinear dependence on
1/lambda_{ab}^2, where lambda_{ab} is the in-plane magnetic penetration depth
in the extrapolated limits T -> 0 and H -> 0. The power coefficient of the
sublinear dependence is close to that determined in severely underdoped YBCO
thin films, indicating that the same relationship between Tc and the superfluid
density is maintained throughout the underdoped regime. The in-plane GL
coherence length (vortex core size) is found to increase with decreasing hole
doping concentration, and exhibit a field dependence that is explained by
proximity-induced superconductivity on the CuO chains. Both the magnetic
penetration depth and the vortex core size are enhanced near 1/8 hole doping,
supporting the belief by some that stripe correlations are a universal property
of high-Tc cuprates.Comment: 12 pages, 13 figure
Comparison of different methods for analyzing SR line shapes in the vortex state of type-II superconductors
A detailed analysis of muon-spin rotation (SR) spectra in the vortex
state of type-II superconductors using different theoretical models is
presented. Analytical approximations of the London and Ginzburg-Landau (GL)
models, as well as an exact solution of the GL model were used. The limits of
the validity of these models and the reliability to extract parameters such as
the magnetic penetration depth and the coherence length from
the experimental SR spectra were investigated. The analysis of the
simulated SR spectra showed that at high magnetic fields there is a strong
correlation between obtained and for any value of the
Ginzburg-Landau parameter . The smaller the applied
magnetic field is, the smaller is the possibility to find the correct value of
. A simultaneous determination of and without any
restrictions is very problematic, independent of the model used to describe the
vortex state. It was found that for extreme type-II superconductors and low
magnetic fields, the fitted value of is practically independent of
. The second-moment method frequently used to analyze SR spectra by
means of a multi-component Gaussian fit, generally yields reliable values of
in the whole range of applied fields
( and are the first and second critical fields, respectively).
These results are also relevant for the interpretation of small-angle neutron
scattering (SANS) experiments of the vortex state in type-II superconductors.Comment: 17 pages, 19 figure
Study of the magnetic penetration depth in RbOs_2O_6
Measurements of the magnetic field penetration depth \lambda in the
pyrochlore superconductor RbOs_2O_6 (T_c\simeq6.3 K) were carried out by means
of the muon-spin-rotation (\muSR) technique. At low temperatures
\lambda^{-2}(T) saturates and becomes constant below T\simeq 0.2T_c, in
agreement with what is expected for weak-coupled s-wave BCS superconductors.
The value of \lambda at T=0 was found to be in the range of 250 nm to 300 nm.
\muSR and equilibrium magnetization measurements both reveal that at low
temperatures is almost (at the level of 10%) independent of the
applied magnetic field. This result suggests that the superconducting energy
gap in RbOs_2O_6 is isotropic.Comment: 8 pages, 9 figure
Developing fencing policies in dryland ecosystems
The daily energy requirements of animals are determined by a combination of physical and physiological factors, but food availability may challenge the capacity to meet nutritional needs. Western gorillas (Gorilla gorilla) are an interesting model for investigating this topic because they are folivore-frugivores that adjust their diet and activities to seasonal variation in fruit availability. Observations of one habituated group of western gorillas in Bai-Hokou, Central African Republic (December 2004-December 2005) were used to examine seasonal variation in diet quality and nutritional intake. We tested if during the high fruit season the food consumed by western gorillas was higher in quality (higher in energy, sugar, fat but lower in fibre and antifeedants) than during the low fruit season. Food consumed during the high fruit season was higher in digestible energy, but not any other macronutrients. Second, we investigated whether the gorillas increased their daily intake of carbohydrates, metabolizable energy (KCal/g OM), or other nutrients during the high fruit season. Intake of dry matter, fibers, fat, protein and the majority of minerals and phenols decreased with increased frugivory and there was some indication of seasonal variation in intake of energy (KCal/g OM), tannins, protein/fiber ratio, and iron. Intake of non-structural carbohydrates and sugars was not influenced by fruit availability. Gorillas are probably able to extract large quantities of energy via fermentation since they rely on proteinaceous leaves during the low fruit season. Macronutrients and micronutrients, but not digestible energy, may be limited for them during times of low fruit availability because they are hind-gut fermenters. We discuss the advantages of seasonal frugivores having large dietary breath and flexibility, significant characteristics to consider in the conservation strategies of endangered species
Muon spin rotation and relaxation in magnetic materials
A review of the muon spin rotation and relaxation (SR) studies on
magnetic materials published from July 1993 is presented. It covers the
investigation of magnetic phase diagrams, of spin dynamics and the analysis of
the magnetic properties of superconductors. We have chosen to focus on selected
experimental works in these different topics. In addition, a list of published
works is provided.Comment: Review article, 59 pages, LaTeX with IoP macro
Spin dynamics and spin freezing in the triangular lattice antiferromagnets FeGa2S4 and NiGa2S4
Magnetic susceptibility and muon spin relaxation (muSR) experiments have been
carried out on the quasi-2D triangular-lattice spin S = 2 antiferromagnet
FeGa2S4. The muSR data indicate a sharp onset of a frozen or nearly-frozen spin
state at T* = 31(2) K, twice the spin-glass-like freezing temperature T_f =
16(1) K. The susceptibility becomes field dependent below T*, but no sharp
anomaly is observed in any bulk property. A similar transition is observed in
muSR data from the spin-1 isomorph NiGa2S4. In both compounds the dynamic muon
spin relaxation rate lambda_d(T) above T* agrees well with a calculation of
spin-lattice relaxation by Chubukov, Sachdev, and Senthil in the renormalized
classical regime of a 2D frustrated quantum antiferromagnet. There is no firm
evidence for other mechanisms. At low temperatures lambda_d(T) becomes
temperature independent in both compounds, indicating persistence of spin
dynamics. Scaling of lambda_d(T) between the two compounds is observed from
~T_f to ~1.5T*. Although the muSR data by themselves cannot exclude a truly
static spin component below T*, together with the susceptibility data they are
consistent with a slowly-fluctuating "spin gel" regime between T_f and T*. Such
a regime and the absence of a divergence in lambda_d(T) at T* are features of
two unconventional mechanisms: (1) binding/unbinding of Z_2 vortex excitations,
and (2) impurity spins in a nonmagnetic spin-nematic ground state. The absence
of a sharp anomaly or history dependence at T* in the susceptibility of
FeGa2S4, and the weakness of such phenomena in NiGa2S4, strongly suggest
transitions to low-temperature phases with unconventional dynamics.Comment: 13 pages, 6 figures, accepted for publication in Physical Review
Observation of two time scales in the ferromagnetic manganite La(1-x)Ca(x)MnO(3), x = 0.3
We report new zero-field muon spin relaxation and neutron spin echo
measurements in ferromagnetic (FM) (La,Ca)MnO3 which taken together suggest two
spatially separated regions in close proximity possessing very different Mn-ion
spin dynamics. One region corresponds to an extended cluster which displays
'critical slowing down' near Tc and an increasing volume fraction below Tc. The
second region possesses more slowly fluctuating spins and a decreasing volume
fraction below Tc. These data are discussed in terms of the growth of small
polarons into overlapping regions of correlated spins below Tc, resulting in a
microscopically inhomogeneous FM transition.Comment: 10 pages, 3 figure
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