高考化学实验的命题趋向及其引起的思考——兼评2002年理科综合第29题（Ⅱ）

化学实验在化学学科中处于举足轻重的地位,它并非以相对独立的形式存在,而是依托于化学知识贯穿于化学的各个领域内。因此,对化学知识的考查,学生能力的考查都离不开化学实验。     

A Molecular Ferroelectric Showing Room‐Temperature Record‐Fast Switching of Spontaneous Polarization

Fast switching of spontaneous polarization (P_s) is one of the most essential requirements for ferroelectrics used in the field of data storage. However, in contrast to inorganic counterparts, the low operating frequency (<500 Hz) for molecular ferroelectrics severely hinders their large‐scale applications. Herein, for the first time, we achieved the room‐temperature fastest switching of the P_s in a new molecular ferroelectric, N‐methylmorpholinium trinitrophenolate ( 1 ), which displays notable ferroelectricity (P_s=3.2 μc cm^?2). Strikingly, electric polarizations of 1 have been switched under a record‐high frequency of 263 kHz, and this performance remains stable without any obvious fatigue after ca. 2×10⁵ switching cycles. To our knowledge, 1 is the first organic ferroelectric to switch polarization at such a high operating frequency, exceeding the majority of organic ferroelectrics, which opens up new possibilities for its potential in the field of non‐volatile memory.     

Preparation and properties of redox responsive modified hyaluronic acid hydrogels for drug release

A series of oxidized hyaluronic acid (oxi‐HA)/3,3′‐dithiobis (propionohydrazide) (DTP) redox responsive hydrogels by Schiff base reaction under physiological conditions were designed and prepared. The influence of the concentration of oxi‐HA and DTP on rheological properties, equilibrium swelling ratio, and degradation rate were investigated. All oxi‐HA/DTP hydrogels exhibited good rheological properties, high equilibrium swelling ratio, low degradation rate, and sustainable drug release properties, and the comprehensive performance of oxi‐HA5/DTP6 hydrogel was better than that of others. The redox responsiveness was evaluated by means of degradation and in vitro bovine serum albumin release behavior investigation with the stimulus of different concentration of dithiothreitol as reducing agent. The intelligent hydrogels could be potentially applied in the fields of drug delivery system, tissue engineering, or cell scaffold materials. Copyright © 2017 John Wiley & Sons, Ltd.     

CDI-promoted direct esterification of P(O)-OH compounds with phenols

A novel and efficient N,N′-carbonyl diimidazole-catalyzed protocol for the direct esterification of P(O)-OH compounds using phenols as efficient esterification reagents is illustrated. It is a simple way to synthesis a broad spectrum of functionalized O-aryl phosphinates, phosphonates, and phosphates from basic starting materials with moderate to excellent yields.     

Pan-connectedness of graphs with large neighborhood unions

Let G be a simple graph with n vertices. For any , let , and , and and u and v has distance 2 in E(G)}. Let l ≥ 1 be an integer. A graph G on n ≥ l vertices is [l, n]-pan-connected if for any , and any integer m with l ≤ m ≤ n, G has a (u, v)-path of length m. In 1998, Wei and Zhu (Graphs Combinatorics 14:263–274, 1998) proved that for a three-connected graph on n ≥ 7 vertices, if NC(G) ≥ n − δ(G) + 1, then G is [6, n]-pan-connected. They conjectured that such graphs should be [5, n]-pan-connected. In this paper, we prove that for a three-connected graph on n ≥ 7 vertices, if NC ₂(G) ≥ n − δ(G) + 1, then G is [5, n]-pan-connected. Consequently, the conjecture of Wei and Zhu is proved as NC ₂(G) ≥ NC(G). Furthermore, we show that the lower bound is best possible and characterize all 2-connected graphs with NC ₂(G) ≥ n − δ(G) + 1 which are not [4, n]-pan-connected.      

二维平板湍流边界层减阻研究

利用一套以位移传感器为主的弹性平衡装置,测量几种不同表面形状设计的平板在充分发展的二维湍流边界层中的减阻效果,并对其减阻机理进行研究。结果表明,大涡破碎器 LEBU(Large Eddy Break-up)和其它减阻装置的形状和布置对表面摩擦阻力有较大影响,在有些设计状态下的平板得到了净减阻。     

Mechanism of field-like torque in spin-orbit torque switching of perpendicular magnetic tunnel junction

Science China Physics, Mechanics & Astronomy - The exact properties of dark matter remain largely unknown despite the accumulating evidence. If dark matter is composed of weakly interacting...     

Kinetic study on extractive resolution of tropic acid by hydroxyethyl-β-cyclodextrin

Reactive extraction is proposed as a promising approach to the resolution of tropic acid (TA), where hydroxyethyl-β-cyclodextrin (HE-β-CD) is used as a selective extractant. The reaction between tropic acid and extractant occurred in the aqueous phase, and kinetic studies of reactive extraction have been investigated in a Lewis-type stirred cell. The batch experiments have been performed to study the process parameters affecting the extraction rate including agitation speed, interfacial area, pH value of aqueous phase, initial concentration of tropic acid enantiomers and initial concentration of extractant on the initial extraction rate. The kinetic equation of the forward rate was deduced at the experiment condition: r _TA+ = (4.041 × 10^?3)[TA₊][CD]²; r _TA? = (3.1234 × 10^?3)[TA_?][CD]².     

Direct synthesis of N-arylamides via the coupling of aryl diazonium tetrafluoroborates and nitriles under transition-metal-free conditions

The direct synthesis of N-arylamides via the coupling of aryl diazonium tetrafluoroborates and nitriles under transition-metal-free conditions has been developed. The reported protocol is practical and represents an efficient method to produce functionalized amides in moderate to good yields.