1,314 research outputs found
On natural resource substitution
We present a simple dynamic model to get some key insights about the substitution of renewable for nonrenewable resources in production and the consequences for sustainability. We highlight the role of the elasticity of substitution (technological component) to determine the adjustment of every sector as a response to scarcity and growing ability of resources (environmental component). Sometimes, the model predicts a smooth substitution of renewable resources for nonrenewables, but this process could work in the opposite direction if renewable resources are temporarily beyond their maximum sustainable yield, so that their marginal natural growth is negative. If substitution possibilities are high enough, it may be optimal to suspend the extraction of a resource, for example, to allow for regeneration of the biomass. We show analytically that a production process is more likely to be sustainable the more heavily it depends on renewable, rather than nonrenewable resources.Renewable resources, Nonrenewable resources, Production, Optimal control.
OPTIMAL SUBSTITUTION OF RENEWABLE AND NONRENEWABLE NATURAL RESOURCES IN PRODUCTION
A theoretical model is presented in order to study the optimal combination of natural resources, used as inputs, taking into account their natural growth ability and the technical possibilities of input substitution. The model enables us to consider renewable resources, nonrenewable, or both. The relative use of resources evolves through time according to the difference between both resources' natural growth and technological flexibility, as measured by the elasticity of substitution of the production function. Output evolves according to a version of the traditional Keynes-Ramsey rule, where the marginal productivity of capital is substituted by the ''marginal productivity of natural capital'', that is a combination of both resources' marginal growth weighted by each resource return in production.Renewable Resources, Nonrenewable Resources, Production, Optimal Control.
Tuning surface metallicity and ferromagnetism by hydrogen adsorption at the polar ZnO(0001) surface
The adsorption of hydrogen on the polar Zn-ended ZnO(0001) surface has been
investigated by density functional {\it ab-initio} calculations. An on top
H(1x1) ordered overlayer with genuine H-Zn chemical bonds is shown to be
energetically favorable. The H covered surface is metallic and spin-polarized,
with a noticeable magnetic moment at the surface region. Lower hydrogen
coverages lead to strengthening of the H-Zn bonds, corrugation of the surface
layer and to an insulating surface. Our results explain experimental
observations of hydrogen adsorption on this surface, and not only predict a
metal-insulator transition, but primarily provide a method to reversible switch
surface magnetism by varying the hydrogen density on the surface.Comment: 4 pages, 3 figure
Dynamical spacetimes and gravitational radiation in a Fully Constrained Formulation
This contribution summarizes the recent work carried out to analyze the
behavior of the hyperbolic sector of the Fully Constrained Formulation (FCF)
derived in Bonazzola et al. 2004. The numerical experiments presented here
allows one to be confident in the performances of the upgraded version of
CoCoNuT's code by replacing the Conformally Flat Condition (CFC) approximation
of the Einstein equations by the FCF.Comment: 4 pages, 7 figures. Accepted for publication in Journal of Physics:
Conference Series, Proceedings of the 8th Edoardo Amaldi Conference on
Gravitational Wave
Magnetism and half-metallicity at the O surfaces of ceramic oxides
The occurence of spin-polarization at ZrO, AlO and MgO
surfaces is proved by means of \textit{ab-initio} calculations within the
density functional theory. Large spin moments, as high as 1.56 , develop
at O-ended polar terminations, transforming the non-magnetic insulator into a
half-metal. The magnetic moments mainly reside in the surface oxygen atoms and
their origin is related to the existence of holes of well-defined spin
polarization at the valence band of the ionic oxide. The direct relation
between magnetization and local loss of donor charge makes possible to extend
the magnetization mechanism beyond surface properties
Towards relativistic simulations of magneto-rotational core collapse
We present a new general relativistic hydrodynamics code specifically designed to study magneto-rotational, relativistic, stellar core collapse. The code is an extension of an existing (and thoroughly tested) hydrodynamics code, which has been applied in the recent past to study relativistic rotational core collapse. It is based on the conformally-flat approximation of Einstein's field equations and conservative formulations for the magneto-hydrodynamics equations. As a first step towards magneto-rotational core collapse simulations the code assumes a passive (test) magnetic field. The paper is focused on the description of the technical details of the numerical implementation, with emphasis on the magnetic field module. A number of code tests are presented and discussed, along with a representative core collapse simulation
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