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Jørgensen, Mathias,Lee, Young-Su,Bjerring, Morten,Jepsen, Lars H.,Akbey, Ü,mit,Cho, Young Whan,Jensen, Torben R. The Royal Society of Chemistry 2018 Dalton Transactions Vol.47 No.46
<P>The new compound Sr(BH4)2(NH3BH3)2 has been synthesized and characterized with <I>in situ</I> powder X-ray diffraction and fast (28 or 60 kHz) magic angle spinning <SUP>1</SUP>H, <SUP>11</SUP>B and <SUP>15</SUP>N NMR and structurally optimized with density functional theory calculations. This investigation reveals complex structural rearrangements for this compound as a function of temperature. A room temperature orthorhombic polymorph, α-Sr(BH4)2(NH3BH3)2, with the space group symmetry <I>Pbca</I>, has been determined with a layered structure of alternating ammonia borane and Sr(BH4)2, partially stabilized by dihydrogen bonding. Surprisingly the crystal symmetry is lowered upon heating, as evidenced both by <I>in situ</I> synchrotron powder X-ray diffraction and <SUP>11</SUP>B MAS NMR data, resulting in an intermediate polymorph, β′-Sr(BH4)2(NH3BH3)2, present from ∼65 to 115 °C. β-Sr(BH4)2(NH3BH3)2, a sub structure of the β′-polymorph showing higher symmetry with the space group symmetry <I>Aba</I>2, forms upon further heating. <I>Ab initio</I> molecular dynamics simulations show that the ammonia borane molecule can dynamically alternate between a bidentate and a tridentate coordination to Sr at finite temperature. The dynamic properties of the ammonia borane molecule in the solid state are suggested to cause the observed structural complexity. Based on simultaneous thermogravimetric analysis, differential scanning calorimetry and mass spectrometry, the decomposition of the compound was investigated showing a stabilization of ammonia borane in the structure relative to other metal borohydride ammonia boranes and neat ammonia borane.</P>
On the nonlinear structural analysis of wind turbine blades using reduced degree-of-freedom models
K. Holm-Jørgensen,J.W. Stærdahl,S.R.K. Nielsen 국제구조공학회 2008 Structural Engineering and Mechanics, An Int'l Jou Vol.28 No.1
Wind turbine blades are increasing in magnitude without a proportional increase of stiffness for which reason geometrical and inertial nonlinearities become increasingly important. Often these effects are analysed using a nonlinear truncated expansion in undamped fixed base mode shapes of a blade, modelling geometrical and inertial nonlinear couplings in the fundamental flap and edge direction. The purpose of this article is to examine the applicability of such a reduced-degree-of-freedom model in predicting the nonlinear response and stability of a blade by comparison to a full model based on a nonlinear co-rotating FE formulation. By use of the reduced-degree-of-freedom model it is shown that under strong resonance excitation of the fundamental flap or edge modes, significant energy is transferred to higher modes due to parametric or nonlinear coupling terms, which influence the response and stability conditions. It is demonstrated that the response predicted by such models in some cases becomes instable or chaotic. However, as a consequence of the energy flow the stability is increased and the tendency of chaotic vibrations is reduced as the number of modes are increased. The FE model representing the case of infinitely many included modes, is shown to predict stable and ordered response for all considered parameters. Further, the analysis shows that the reduced-degree-of-freedom model of relatively low order overestimates the response near resonance peaks, which is a consequence of the small number of included modes. The qualitative erratic response and stability prediction of the reduced order models take place at frequencies slightly above normal operation. However, for normal operation of the wind turbine without resonance excitation 4 modes in the reduced-degree-of-freedom model perform acceptable.
Technical Barriers, Import Licenses and Tariffs as Means of Limiting Market Access
( Jan G. Jørgensen ),( Philipp J. H. Schroder ) 세종대학교 경제통합연구소 (구 세종대학교 국제경제연구소) 2006 Journal of Economic Integration Vol.21 No.1
Technical barriers (standards), import licenses and tariffs may be deployed as means of limiting the market access of foreign firms. The present paper examines these measures in a setting of monopolistic competition. We find that, if protection focuses predominantly on the number of foreign firms accessing the domestic market, a technical barrier (an import license) may dominate a tariff (tariff and a technical barrier) in terms of consumer welfare, even when tariff revenues are fully redistributed. However, if protection pays sucfficient focus on limiting the total import volume, then tariffs are the preferred means of protection. Within the model, reductions in technical barriers and tariffs, the removal of licensing schemes, and a harmonization of standards are all welfare-improving policies.
Market Integration and Industrial Specialization on a Monopolistic Competitive Market
Hansen, JØrgen Drud,JØrgensen , Jan Guldager 세종대학교 국제경제연구소 2001 Journal of Economic Integration Vol.16 No.3
This paper examines the relationship between market integration and product diversification in a Chamberlinian model of monopolistic competition. In the first version of the model, production of the firm is organised in activities producing either one or two horizontally differentiated product-variants. The cost functions show both scale and scope economies. Market integration is illustrated by an increase in the market size. For increasing market size, each firm shifts from producing tow variants to producing one variant only at a certain threshold value of market size. Passing this threshold value the firm size measured by total output changes discontinuously leaving the effect on firm size ambiguous. For specific specification of the perceived demand of the individual firm hysteresis of the industrial structure may appear in the sense that the threshold value of the market size for shifting from two to one variant production exceeds that of the threshold value of market size of shifting from one to two variants. In the last part of the paper, the model is generalised to a continuum of variants and it is shown that an increase of the market size reduces the number of variants produced by each firm, whereas the hysteresis phenomenon disappears.
Market Integration and Industrial Specialization on a Monopolistic Competitive Market
( Jørgen Drud Hansen ),( Jan Guldager Jørgensen ) 세종대학교 경제통합연구소 2001 Journal of Economic Integration Vol.16 No.3
This paper examines the relationship between market integration and product diversification in a Chamberlinian model of monopolistic competition. In the first version of the model, production of the firm is organised in activities producing either one or two horizontally differentiated product-variants. The cost functions show both scale and scope economies. Market integration is illustrated by an increase in the market size. For increasing market size, each firm shifts from producing two variants to producing one variant only at a certain threshold value of market size. Passing this threshold value the firm size measured by total output changes discontinuously leaving the effect on firm size ambiguous. For specific specification of the perceived demand of the individual firm hysteresis of the industrial structure may appear in the sense that the threshold value of the market size for shifting from two to one variant production exceeds that of the threshold value of market size of shifting from one to two variants. In the last part of the paper, the model is generalised to a continuum of variants and it is shown that an increase of the market size reduces the number of variants produced by each firm, whereas the hysteresis phenomenon disappears.