价格波动对进出初级产品行业的影响

初级产品生产行业有两大特点：一是其初级投资有较大的不可逆转性，二是其产品价格具有较大的随机性（不确定性）。这两点的同时存在，给进入和退出初级产品行业造成壁垒。     

Carnot efficiency: Why?

In 1824, Carnot proposed a cycle operating on reversibility principles. He proved that there exists an upper limit of the efficiency of this cycle and this limit is also the upper limit for any real process. The irreversibility related to the finite-time and the finite-size constraints are fundamental for the optimization of any real thermodynamic system. It has been pointed out how fundamental is the interaction between any open system and its surroundings. The meaning of the Carnot efficiency is that even in the ideal condition, when there is no dissipation, there exists something that does not allow the system to convert all the energies absorbed in work. The aim of this paper is to show why this happens, starting from a variational approach of thermodynamics.     

Superconducting characteristics of (Bi, Pb)-2223 single crystals with controlled oxygen-content

Critical current density was measured for oxygen-controlled (Bi, Pb)-2223 single crystals before and after the irradiation with gold ions in a magnetic field parallel to the irradiation-induced defects along the c-axis. Eleven specimens prepared in different annealing conditions were measured. The condensation energy density of each specimen was evaluated from the observed critical current density by using the summation theory of pinning forces of columnar defects and the flux creep theory. It was found that the specimen heat-treated at 1 atm in oxygen atmosphere has the highest condensation energy density among all specimens. Hence, it is speculated that the optimum oxygen pressure for the anneal is around 1 atm.     

Generic guiding principle of HTSC material design for high-field application

Through the analyses on the irreversibility lines (ILLs) under H||c for various high-T_c superconducting (HTSC) single-crystalline samples, we found two universal functions representing ILLs. In these functions, the electromagnetic anisotropy factor γ² plays a crucial role. The appropriate chemical substitution, such as Bi(Pb)2212, is an effective method to improve flux pinning properties via decreasing γ².     

Hysteresis loops predicted by isoenergy density theory for polycrystals. Part II: cyclic heating and cooling effects predicted from non-equilibrium theory for 6061-T6 aluminum, SAE 4340 steel and Ti–8Al–1Mo–1V titanium cylindrical bars

Mathematical tools and physical models should fit like hand and glove. Traditionally, such an understanding has worked surprisingly well in mechanics and physics. Differential calculus enabled the determination of motion of celestial bodies. Heisenberg's use of Hilbert space applied to calculations in quantum mechanics. These foot steps, however, failed to continue in the field of material science when attempting to address the evolution of material damage. On one hand, the mathematical formulation of the Riemann–Hilbert problem achieved great success for solving macrocrack problems but whether the same tool could be used for lower scale defects seems to be of less concern. This is a surprise because it is not obvious by any means whether microcracks can be distinguished from macrocrack by size difference alone, particularly when they are both treated in a single formulation.There are several issues of the fatigue problem that must be addressed. To begin with, local failure initiating near a surface should be distinguished from global failure that correspond to separation of a specimen into two or more pieces. The interim stages of initiation and termination would depend on the ductility and brittleness of the material in addition to the history of cyclic loading. Three different materials 6061-T6 aluminum, SAE 4340 steel and Ti–8Al–1Mo–1V titanium will therefore be analyzed to show how their uniaxial properties would compare with the dissipation energy density functions and oscillation in temperature. Hysteresis loops for the local material points are determined. Their averages for the ASTM standard hour-glass specimen are then found. They give the global hysteresis loops corresponding to measurements from the uniaxial tests. The local dissipative energy density loops represent the irreversible nature of the materials are also taken into consideration. They are calculated for the soft aluminum and the hard titanium with steel being in between.Obtained also are the cyclic cooling and heating characteristics of the material under fatigue; they fluctuate about the ambient temperature. The -function designating the order and disorder of the fatigue process is also found to be oscillatory in character. Unlike entropy, it is not non-increasing or non-decreasing. It can change sign in a given process. This implies that the order of a system can be enhanced and then impeded or vice versa. Such a behavior is shown to prevail during the initial stage of the fatigue.     

OPTIMAL REGULATION OF NATURAL RESOURCES IN THE PRESENCE OF IRREVERSIBLE THRESHOLD EFFECTS

ABSTRACT. Thresholds are an important physical consideration in the regulation of natural resources. This paper is the first to present a complete analytical framework in which to evaluate optimal regulation of a natural resource in the presence of irreversible threshold effects. Necessary conditions are presented for optimal regulation of these problems both for when the threshold has a known location in state‐space and for when the location of the threshold is unknown. In the case where the location is known, the literature is corrected on a seemingly minor technical point regarding the behavior of the co‐state variables that turns out to be of considerable importance. For the case when the location of the threshold is not known, it is shown that thresholds in state‐space implies a nonstandard risk structure.