Kondratiev, a Russian economist, studied 19th and early 20th century price data and concluded that economies moved through year cycles, with an upswing followed by a period of stagnation which gives way to a downswing. His waves — although of different length — are also sometime described as a four-phase cycle of Prosperity-Recession-Depression-Recovery. Although his name is associated with the waves, he was building on the work of Dutch economists. These cycles are typically longer.
And one of the open research questions for me is the extent to which we might have to think about cycles of different lengths interacting in different ways. One last thought from me: the patterns of change described here all follow general patterns of systemic change. If one thinks of the Panarchy model , developed by Gunderson and Hollings from studying the life of forests, it has a cycle that also goes through four stages: Expansion or Exploitation -Conservation-Release-Reorganisation.
At the very least it seems reasonable to believe that human and social systems might follow similar systemic patterns. You are commenting using your WordPress. You are commenting using your Google account. You are commenting using your Twitter account. You are commenting using your Facebook account. Notify me of new comments via email. Notify me of new posts via email. Search for: Search. Date: 5 May, Author: thenextwavefutures 2 Comments. Other gaped materials below the dot middle line are shown with half filled orange squares or pink stars.
To evaluate the results, a standard to choose the most possible candidates is defined. Considering the average discrepancy The pink stars highlight the present used commercial products. If only the and B v are used as criteria, it seems substantial materials are available. However, the zero-band-gap material is not suitable for windows due to the feasible IR absorption by their intrinsic free carriers generated at the elevated temperature. Thus, a further filtration is necessary through band structure evaluation for the candidates in LWP and LWF cover regions.
Here, the materials that show semimetallic or metallic characteristics are excluded see the purple circles in the inset of Fig. According to a rule-of-thumb by Hilton 23 , a material should have a band gap larger than 1. In other words, these candidates except TiSe should have the abilities to suppress the absorption of infrared radiation at a relatively high temperature.
Stability is another important factor for the application of the new potential candidates.
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The dynamic stabilities for all the seven candidates are clearly demonstrated without any imaginary frequency. All these candidates hold significant electronic band gap. These show their stability at relatively low temperature. The Fermi Level is marked with red dashed lines in band structures. To further examine the applicability of these IR candidates in harsh external environment such as at the very high temperature, molecular dynamic simulations are carried out for these compounds. High-temperature molecular dynamics HTMD methods are widely used to test the thermal stability and behavior in materials The evolution of average bond length is analyzed during the high-temperature annealing.
As a reference, ZnS shows a relatively good thermal stability with harmonic variation of the bond length. In fact, very little distortive structure after the annealing also supports the stability. In contrast, TiSe and HgF 2 are seriously distorted after the annealing see their final structures. Especially, for TiSe, the bond length fluctuates significantly.
The final snapshot of the crystal structure after ps annealing is shown together with its bond-length evolution. Figure 5 a shows the average long-wave absorption limit , i. For halogen compounds, the average varies from For chalcogen compounds, the average from 9. However, for nitrogen compounds, the is the smallest from 7. Considering their mechanical performance, chalcogen compounds should be the best platform for IR window candidates.
Most recently, a copolymer with a very good IR transmission has been produced with inverse vulcanization, where molten sulphur, acting as a solvent, was copolymerized with 1,3-diisopropenylbenzene DIB to prepare a chemically stable and processable sulphur plastic Thus, this IR material gene picture may offer some usual references for the future industrial design. Both the minimum and maximum are shown to see the range of the. In this work, a simple and efficient method has been proposed to explore the potential LWIR window materials with both the suitable mechanical and optical performances by a first-principles gene-like searching.
The two descriptors, namely and B v , well describe the properties for IR window materials. Especially, in the benchmark test, the theoretical simulations are well consistent with experiments. Especially, the performances of TiS and CdF 2 can be comparable to the most popular ZnS at high temperature and in chemical stability. Here, we stress that the development of a practical IR window is in fact a big engineering project. To meet the requirement, people must carefully control the defects during the growth of material because defects or unintentional impurities can always lead to IR absorptions due to the local state inside the band gap or the free carrier supplement at band edge.
Also because some large scale defects such as boundary, void or dislocation can scatter IR seriously, thus worsen the transmission. On the other hand, some cover layers are also required to protect the IR window materials from the erosion by wind and rain. At this stage, we still have not considered the problems mentioned above, however the present strategy at least tells people which candidate should be focused on for IR window materials.
We believe the same idea can also be used in searching other materials toward a special performance with the help of material gene-like exploration. Phonon dispersion and elastic constants are obtained to estimate the long-wave absorption limit and bulk modulus for two hundred and fifty-three kinds of A x B y binary compounds, based on first-principles calculations.
The projector augmented wave PAW pseudopotentials 40 are used to describe electron-ion interactions. The energy cutoff for the plane wave expansion is chosen equal to or larger than 1. Lattice geometry relaxation is run until all the forces are smaller than 0. Here, the phonon dispersion calculation uses the supercell method by phonopy code Bulk modulus B v Voigt-type for cubic, hexagonal, or tetragonal lattice is calculated from the elastic constant C ij by Voigt-Reuss-Hill approximations How to cite this article : Du, J.
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