25,489 research outputs found
Prenatal Smoking Cessation and Infant Health: Evidence from Sibling Births
This paper uses a unique large panel data of sibling births to provide new evidence on when prenatal smokers must quit smoking to deliver the healthy newborn. In a series of the mother fixed effect estimation, I find robust results that early cessation in the first trimester nullifies the adverse smoking impact, but late cessation in the second trimester still leaves a remarkably irreversible damage on infant health. About two third of the adverse smoking impact on infant health occurs in the second trimester, mainly through fetal growth retardation. Therefore the first trimester is the critical period for prenatal smoking cessation. In particular, failing to stop smoking promptly in this period is crucial to explain why low socioeconomic status prenatal smokers transmit their poor health and economic status to the offspring. The policy implication is that reallocating resources on prenatal smoking cessation towards the first trimester can lead to a significant effciency gain. This paper also uncovers a new source of downward bias in estimating the causal relation between a group measure "prenatal smokers" and infant health, if the timing information of smoking cessation is misused. Key Words: Prenatal Smoking, Timing of Smoking Cessation, Birth Weight, Low Birth Weight
Development of a high strength Al-Mg2Si-Mg-Zn based alloy for high pressure die casting
A high strength Al-Mg2Si-Mg-Zn based alloy has been developed for the application in high pressure die casting to provide improved mechanical properties. The effect of various alloying elements on the microstructure and mechanical properties including yield strength, ultimate tensile strength and elongation of the alloy was investigated under the as-cast and heat-treated conditions. The typical composition of the high strength alloy has been optimised to be Al-8.0wt%Mg2Si-6.0wt%Mg-3.5wt%Zn-0.6wt%Mn (Al-11.0wt%Mg-2.9wt%Si-3.5wt%Zn-0.6wt%Mn) with unavoidable trace impurities. The mechanical properties of the alloy were enhanced by a quick solution treatment followed by ageing treatment. The improved tensile properties were at a level of yield strength over 300MPa, the ultimate tensile strength over 420MPa and the elongation over 3% assessed using international standard tensile samples made by high pressure die casting. The microstructure of the die-cast alloy consisted of the primary α-Al phase, Al-Mg2Si eutectics, AlMgZn intermetallics and α-AlFeMnSi intermetallics under the as-cast condition. The AlMgZn intermetallic compound was dissolved into the Al-matrix during solution treatment and subsequently precipitated during ageing treatment for providing the effective improvement of the mechanical properties.The financial support is gratefully acknowledged for the Engineering and Physical Sciences Research Council (EPSRC) (Project number: EP/I038616/1), Technology Strategy Board (TSB) (Project number: 101172) and Jaguar Land Rover (JLR), United Kingdom
Asymptotics in directed exponential random graph models with an increasing bi-degree sequence
Although asymptotic analyses of undirected network models based on degree
sequences have started to appear in recent literature, it remains an open
problem to study statistical properties of directed network models. In this
paper, we provide for the first time a rigorous analysis of directed
exponential random graph models using the in-degrees and out-degrees as
sufficient statistics with binary as well as continuous weighted edges. We
establish the uniform consistency and the asymptotic normality for the maximum
likelihood estimate, when the number of parameters grows and only one realized
observation of the graph is available. One key technique in the proofs is to
approximate the inverse of the Fisher information matrix using a simple matrix
with high accuracy. Numerical studies confirm our theoretical findings.Comment: Published at http://dx.doi.org/10.1214/15-AOS1343 in the Annals of
Statistics (http://www.imstat.org/aos/) by the Institute of Mathematical
Statistics (http://www.imstat.org
The silicate model and carbon rich model of CoRoT-7b, Kepler-9d and Kepler-10b
Possible bulk compositions of the super-Earth exoplanets, CoRoT-7b,
Kepler-9d, and Kepler-10b are investigated by applying a commonly used silicate
and a non-standard carbon model. Their internal structures are deduced using
the suitable equation of state of the materials. The degeneracy problems of
their compositions can be partly overcome, based on the fact that all three
planets are extremely close to their host stars. By analyzing the numerical
results, we conclude: 1) The iron core of CoRoT-7b is not more than 27% of its
total mass within 1 mass-radius error bars, so an Earth-like
composition is less likely, but its carbon rich model can be compatible with an
Earth-like core/mantle mass fraction; 2) Kepler-10b is more likely with a
Mercury-like composition, its old age implies that its high iron content may be
a result of strong solar wind or giant impact; 3) the transiting-only
super-Earth Kepler-9d is also discussed. Combining its possible composition
with the formation theory, we can place some constraints on its mass and bulk
composition.Comment: 20 pages, 8figures, accepted for publication in RAA. arXiv admin
note: text overlap with arXiv:0707.289
Strain-induced energy band gap opening in two-dimensional bilayered silicon film
This work presents a theoretical study of the structural and electronic
properties of bilayered silicon films under in-plane biaxial strain/stress
using density functional theory. Atomic structures of the two-dimensional
silicon films are optimized by using both the local-density approximation and
generalized gradient approximation. In the absence of strain/stress, five
buckled hexagonal honeycomb structures of the bilayered silicon film have been
obtained as local energy minima and their structural stability has been
verified. These structures present a Dirac-cone shaped energy band diagram with
zero energy band gaps. Applying tensile biaxial strain leads to a reduction of
the buckling height. Atomically flat structures with zero bucking height have
been observed when the AA-stacking structures are under a critical biaxial
strain. Increase of the strain between 10.7% ~ 15.4% results in a band-gap
opening with a maximum energy band gap opening of ~168.0 meV obtained when
14.3% strain is applied. Energy band diagram, electron transmission efficiency,
and the charge transport property are calculated.Comment: 18 pages, 5 figures, 1 tabl
- …