多晶Ni-Mn-Ga磁性记忆合金的相变行为及稀土元素铽的作用

研究了Ni50Mn25 xGa25-x 和Ni50Mn29Ga21-xTbx 2种成分系列磁性记忆合金的相变行为. 保持Ni含量不变, 增加Mn, 降低Ga含量会使马氏体相变温度明显提高, 同时相变滞后温区减小, 居里温度基本不变. 如果添加稀土元素铽, 相变温度继续升高, 居里温度仍然不变, 材料继续保持强的铁磁性及热弹性马氏体相变的特征.     

磁性高分子链的磁性质和构象性质的Monte Carlo模拟

采用退火 (Annealing)MonteCarlo方法 ,从高温到低温顺序模拟了简立方格点上考虑最近邻Ising相互作用的磁性高分子链在不同温度的磁性质和构象性质 .磁性高分子链在低温下存在自发磁矩 ,无限长链的临界温度Tc=1 77± 0 0 5J kB.在临界温度附近 ,高分子链经历了从伸展的无规线团到紧缩球体的塌缩相变 .对链的尺寸、形状、近邻数及能量的分析表明 ,高分子链的构象性质从温度Tc=1 77开始发生较明显的变化 ,这表明高分子Ising链的相变是Ising相互作用和链节运动协同作用的结果 .     

低温度系数的Nd-Fe-B永磁合金

研究了取代元素钴、镝、钬、铒、钆等对Nd-Fe-B系永磁合金磁性及热稳定性的影响。用钻和重稀土元素取代铁和钕,合金居里温度升高,磁感总损失下降,平均可逆温度系数下降最后变为正值。已得到一种温度系数α=-0.028％/℃(20～100℃)的Nd Fe B系永磁合金,它可以较好地工作在150℃的环境中。用这种取代方法获得的温度系数为零的合金,其磁性为:Br=7900G,_iH_c>16kOe,_bH_c=7.3kOe,(BH)_(max)=14.3MGOe。     

对位取代苯甲酸对苯二酯类液晶相变和取代基作用的DSC研究

用准确可靠的DSC法研究了对位取代苯甲酸对苯二酯类的五个仔细纯化过的液晶化合物,得到了相变的温度、焓及熵的数据.据此给出液晶性质和分子结构关系的规律.指出不能用分子增宽导致分子间色散引力减弱的效应解释溴取代侧基的影响,相变温度的降低是熵效应的结果.用分子对称性和S_c、N相稳定性的观点解释了S_c相消失和相变温度降低的现象.从分子对称性和氰基极性观点讨论了中心苯环2,3-位氰基取代基对T_m和T_c的影响;指出烷氧基比烷基更有利于向列相的热稳定性.     

钆替代对ErGa化合物磁性和磁热性能的影响

磁热效应材料可用作磁制冷,因而引起研究人员的广泛关注。为了提高Er Ga材料的磁热效应性能,用高自旋重稀土Gd原子替代Er Ga化合物中的Er原子,系统研究了Gd替代对其结构、磁性、磁熵变的影响。粉末中子衍射及粉末X射线衍射实验证实,当Gd原子替代量从x=0到1.0时,不改变化合物的晶体结构。磁性测试结果表明,Gd原子的替代不同程度地改变了该材料体系的自旋重取向相变温度及铁磁-顺磁相变温度。基于相变温度的改变,Gd替代对材料的磁熵变峰值、制冷温跨及制冷能力也产生显著影响。Gd替代量x=0.1,0.2,0.3,0.4时,磁熵变温度关系曲线呈现平台状特征,尤其是Gd替代量x=0.2和0.3时,材料的综合性能较好。     

含二茂铁的双取代哒嗪的合成和介晶性

合成了8个未见报道的中心桥连基为哒嗪环、酯基、CHN基,含有二茂铁、三个苯环、不同末端链长度的双取代哒嗪化合物,并通过示差扫描量热仪(DSC)对其介晶性进行了表征.研究表明,末端链长度对相变温度和清亮点温度均有影响,但对相变温度范围影响较小.     

含西佛碱的3,6-双取代哒嗪的合成和介晶性

合成了8个中心桥连基为哒嗪环、酯基、CH=N基,含有胆甾基、三个苯环、不同未端链长度的双取代哒嗪化合物,并通过示差扫描量热法（DSC）对其介晶性进行了表征。研究表明,末端链长度对相变温度和清亮点温度均有影响,但对相变温度范围影响小。     

含胆甾基、西佛碱的3,6-双取代哒嗪的合成和介晶性

合成了8个未见报道的中心桥连基为酯基、CH＝N基, 含有哒嗪环、胆甾基、二个苯环、不同末端链长度的双取代哒嗪化合物, 并通过示差扫描量热法(DSC)对其介晶性进行了表征. 研究表明, 末端链长度对相变温度和清亮点温度均有影响, 但对相变温度范围影响较小.     

含胆甾基的双取代哒嗪的合成和介晶性

合成了8个未见报道的中心桥连基为哒嗪环、酯基、CH＝N基,含有胆甾基、二个苯环、不同末端链长度的双取代哒嗪化合物,并通过示差扫描量热法(DSC)对其介晶性进行了表征.研究表明,末端链长度对相变温度和清亮点温度均有影响,但对相变温度范围影响较小.     

Sm2Co17型永磁合金热失效的扫描电子显微镜分析

高温度稳定型Sm2Co17永磁体的磁性能会随着使用温度的提高有所下降.为了探究高温对合金微观结构的影响,对该材料进行差热分析后得到3个明显的相变温度点:254、342和540℃.然后取同一批次的该磁体在上述温度下分别进行热处理30 min,并使用扫描电子显微镜和X射线能谱仪对处理后样品进行显微结构和成分分析.结果表明,Cu和Fe的析出和氧化是材料磁性能失效的原因,分析结果对预防该类材料失效提供了微观依据.     

Aminolysis of Y-substituted phenyl X-substituted cinnamates and benzoates: effect of modification of the nonleaving group from benzoyl to cinnamoyl

A kinetic study is reported for reactions of Y-substituted phenyl X-substituted cinnamates (1a-e and 3a-g) and benzoates (2a-e and 4a-g) with a series of alicyclic secondary amines in 80 mol % H2O/20 mol % DMSO at 25.0 +/- 0.1 degrees C. Reactions of 2,4-dinitrophenyl X-substituted cinnamates (1a-e) and benzoates (2a-e) with amines result in linear Yukawa-Tsuno plots. The rho(X) values are much smaller for the reactions of 1a-e than for those of 2a-e. A distance effect and the nature of the reaction mechanism (i.e., a concerted mechanism for 1a-e) have been suggested to be responsible for the small rho(X) values. The Br?nsted-type plots for the reactions of 2,4-dinitrophenyl X-substituted cinnamates (1a, 1c, and 1e) with amines are curved with a decreasing betanuc value from 0.65 to 0.3-0.4. The reactions of Y-substituted phenyl cinnamates (3a-g) with morpholine also result in a curved Br?nsted plot, while the corresponding reactions of Y-substituted phenyl benzoates (4a-e) exhibit a linear Br?nsted plot. It has been concluded that the curved Br?nsted plots found for the reactions of the cinnamates (1a, 1c, 1e, and 3a-g) are not due to a change in the rate-determining step (RDS) but due to a normal Hammond effect for a concerted mechanism, that is, an earlier transition state (TS) for a more reactive amine or substrate.     

Changes in Electron Structure of the Triple Bond in Substituted Acetylene and Diacetylene Derivatives

The substituent effect is usually considered by means of various Hammett-like substituent constants and is most often related to aromatic systems. Unlike this, we present results of our research on the influence of 27 substituents spanning a wide range of electronic properties, from strongly electron-withdrawing to strongly electron-donating, on the electron structure of X-substituted acetylenes and diacetylenes – thus the systems which until now have practically not been subject of any deeper studies. It is shown that the interaction through triple bond(s) is associated with a significant advantage of resonance effects and that the substituent effect transmitted by the C≡C−C≡C unit is about half of that transmitted by the C≡C unit alone. Substituent X mainly affects the closest carbon atom by means of proximity effect, hence changes of charge on this atom do not follow any substituent constants. The effect on further carbon atoms is much smaller. The presence of the C≡C−C≡C unit withdraws more charge from X than a triple bond alone, and hinders communication between X and the terminal H atom. Comparison of substituent effects to those present in X-substituted benzene derivatives shows that the electronic properties of the terminal hydrogen atom in acetylenes and diacetylenes are most similar to the electronic properties of ortho and para hydrogen atoms in X-substituted benzene derivatives.     

Substituent effects in IR spectroscopy—XII. Field effects of 4-substituents on the cyclopropyl ν(CH) vibrator of tricyclenes

The variation of the equivalent C2H and C6H force constants of 4X-substituted tricyclenes depends mainly on the electrostatic forces exerted by the C4X dipole. The sensitivity of the cyclopropanic bonds to electronic substituent effects in these tricyclenes is compared to the substituent dependence of ν(CH) in other systems.     

The construction of graph models for calculations of the properties of substitution isomers of basis structures on the basis of additivity of energy contributions

A method for the construction of additive models for calculations of the properties of substitution isomers of basis structures is described for the example of a series of X-substituted methylsilanes CH_{3 − k} X _k -SiH_{3 − l} X _l (where X = CH₃, F, Cl, …, k, l = 0, 1, 2, 3). The method is based on similarity of subgraphs in graphs of several molecules and the arrangement of polygonal numbers (triangular, tetrahedral) of the Pascal triangle. Parameters taking into account multiple nonvalence interactions (-C-Si<, >C-Si<, …) through two atoms along the molecular chain of an X-substituted methylsilane (X = CH₃) were for the first time explicitly included in the calculation scheme. Taking these interactions into account allows us to completely differentiate all the structural isomers of certain molecules and obtain numerical parameter values for predicting properties P under consideration in various approximations. Numerical calculations of Δ_f H _{g,298 K}^o were performed for 16 alkylsilanes (as X-substituted methylsilanes), including 7 compounds not studied experimentally.     

Effect of o-methyl group on rate, mechanism, and resonance contribution: aminolysis of Y-substituted phenyl X-substituted 2-methylbenzoates

Second-order rate constants have been determined spectrophotometrically for the reactions of 4-nitrophenyl X-substituted 2-methylbenzoates (2a-e) and Y-substituted phenyl 2-methylbenzoates (3a-e) with alicyclic secondary amines in 80 mol % H(2)O/20 mol % DMSO at 25.0 +/- 0.1 degrees C. The o-methyl group in the benzoyl moiety of 2a-e retards the reaction rate but does not influence the reaction mechanism. The Hammett plots for the reactions of 2a-e are nonlinear, while the corresponding Yukawa-Tsuno plots are linear with large r values (1.06-1.70). The linear Yukawa-Tsuno plots suggest that stabilization of the ground-state through resonance interaction between the electron donating substituent X and the carbonyl group is responsible for the nonlinear Hammett plots, while the large r values imply that the ground-state resonance interaction is significant. The reactions of 2a-e resulted in smaller rho(X) values but larger r values than the corresponding reactions of 4-nitrophenyl X-substituted benzoates (1a-e). The small rho(X) value for the reactions of 2a-e (e.g., rho(X) = 0.22) is suggested to be responsible for the large r value (e.g., r = 1.70). The reactions of 3a-e with piperidine are proposed to proceed in a stepwise manner with a change in the rate-determining step on the basis of the curved Br?nsted-type plot obtained. Microscopic rate constants associated with the reactions of 3a-e are also consistent with the proposed mechanism.     

Regioselectivity and the nature of the reaction mechanism in nucleophilic substitution reactions of 2,4-dinitrophenyl X-substituted benzenesulfonates with primary amines

Second-order rate constants have been measured for the reaction of 2,4-dinitrophenyl X-substituted benzenesulfonates with a series of primary amines. The nucleophilic substitution reaction proceeds through competitive S-O and C-O bond fission pathways. The S-O bond fission occurs dominantly for reactions with highly basic amines or with substrates having a strong electron-withdrawing group in the sulfonyl moiety. On the other hand, the C-O bond fission occurs considerably for the reactions with low basic amines or with substrates having a strong electron-donating group in the sulfonyl moiety, emphasizing that the regioselectivity is governed by both the amine basicity and the electronic effect of the sulfonyl substituent X. The apparent second-order rate constants for the S-O bond fission have resulted in a nonlinear Br?nsted-type plot for the reaction of 2,4-dinitrophenyl benzenesulfonate with 10 different primary amines, suggesting that a change in the rate-determining step occurs upon changing the amine basicity. The microscopic rate constants (k(1) and k(2)/k(-)(1) ratio) associated with the S-O bond fission pathway support the proposed mechanism. The second-order rate constants for the S-O bond fission result in good linear Yukawa-Tsuno plots for the aminolyses of 2,4-dinitrophenyl X-substituted benzenesulfonates. However, the second-order rate constants for the C-O bond fission show no correlation with the electronic nature of the sulfonyl substituent X, indicating that the C-O bond fission proceeds through an S(N)Ar mechanism in which the leaving group departure occurs rapidly after the rate-determining step.     

Rapid separation of VI/VII technetium oxidation states by solvent extraction with iodonitrotetrazolium chloride

Technetium⁹⁹ poses a difficult problem at many nuclear waste disposal sites, as there have been multiple incidents of its release to the environment due to large quantities of fission products disposed in storage tanks. Tc is mostly present under two oxidation states, Tc(VII) and Tc(IV) and the separation of Tc(IV) from Tc(VII) is often crucial for laboratory-scale work performed for the study of Tc. This work offers a method for the rapid separation of Tc(IV) from Tc(VII), using a solvent extraction system containing iodonitrotetrazolium chloride and chloroform.     

A polylinear regression model taking into account the cross effects of the structure and temperature in the reactions of phenyloxirane with benzoic acids, catalyzed by pyridines: Evidence of the isoparametricity phenomenon

The joint effect of the structure and temperature on the rate of the reactions of phenyloxirane with Y-substituted benzoic acids in acetonitrile, catalyzed by X-substituted pyridines, was studied. A polylinear regression model that adequately describes the effects of cross-varied factors was calculated. Owing to intensive interaction of the effects of the substituents X and temperature, experimental evidence for the isoparametricity phenomenon was obtained.     

Schemes of calculation of the properties of substituted ethylenes

Calculation schemes for estimation of the properties of X-substituted ethylenes based on the concept of atoms interacting in pairs are described. The equivalence of these schemes are shown, and the relations between their parameters are established.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 2586–2587, October, 1996.     

(99)Tc and re incorporated into metal oxide polyoxometalates: oxidation state stability elucidated by electrochemistry and theory

The radioactive element technetium-99 ((99)Tc, half-life = 2.1 × 10(5) years, β(-) of 253 keV), is a major byproduct of (235)U fission in the nuclear fuel cycle. (99)Tc is also found in radioactive waste tanks and in the environment at National Lab sites and fuel reprocessing centers. Separation and storage of the long-lived (99)Tc in an appropriate and stable waste-form is an important issue that needs to be addressed. Considering metal oxide solid-state materials as potential storage matrixes for Tc, we are examining the redox speciation of Tc on the molecular level using polyoxometalates (POMs) as models. In this study we investigate the electrochemistry of Tc complexes of the monovacant Wells-Dawson isomers, α(1)-P(2)W(17)O(61)(10-) (α1) and α(2)-P(2)W(17)O(61)(10-) (α2) to identify features of metal oxide materials that can stabilize the immobile Tc(IV) oxidation state accessed from the synthesized Tc(V)O species and to interrogate other possible oxidation states available to Tc within these materials. The experimental results are consistent with density functional theory (DFT) calculations. Electrochemistry of K(7-n)H(n)[Tc(V)O(α(1)-P(2)W(17)O(61))] (Tc(V)O-α1), K(7-n)H(n)[Tc(V)O(α(2)-P(2)W(17)O(61))] (Tc(V)O-α2) and their rhenium analogues as a function of pH show that the Tc-containing derivatives are always more readily reduced than their Re analogues. Both Tc and Re are reduced more readily in the lacunary α1 site as compared to the α2 site. The DFT calculations elucidate that the highest oxidation state attainable for Re is VII while, under the same electrochemistry conditions, the highest oxidation state for Tc is VI. The M(V)→ M(IV) reduction processes for Tc(V)O-α1 are not pH dependent or only slightly pH dependent suggesting that protonation does not accompany reduction of this species unlike the M(V)O-α2 (M = (99)Tc, Re) and Re(V)O-α1 where M(V/IV) reduction process must occur hand in hand with protonation of the terminal M═O to make the π*(M═O) orbitals accessible to the addition of electrons. This result is consistent with previous extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge structure (XANES) data that reveal that the Tc(V) is "pulled" into the -α1 framework and that may facilitate the reduction of Tc(V)O-α1 and stabilize lower Tc oxidation states. This study highlights the inequivalency of the two sites, and their impact on the chemical properties of the Tc substituted in these positions.