共查询到20条相似文献,搜索用时 171 毫秒
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通过真空电弧熔炼、长时间真空热处理的方法获得了单相NaZn13-型LaFe13-xSix(1.2≤x≤2.2)化合物,并测量了它们的磁化强度与磁场和温度的关系,用Maxwell关系式计算出在不同磁场下化合物的熵变ΔS。用Landau的二级相变理论及平均场近似下的标度律,分析拟合了LaFe13-xSix化合物ΔS与H之间的关系,对于具有一级和二级相变的材料,发现均存在ΔS∝H2/3的关系,只是拟合得到的参数不同。采用熵变峰值拟合得到的参数能够反映材料中一级磁性相变的程度,研究磁场诱导的熵变与磁场的关系可以为磁制冷研制提供指导。 相似文献
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可逆型化学振荡器 总被引:2,自引:0,他引:2
该文给出了可逆型化学振荡器的定义、意义和可能存在的类型。这些类型包括:(1)自发乳化,通电破乳;(2)乳液中自发产生的甘油三酯水解振荡反应及通电逆向酯化反应;(3)用表面活性剂有机溶液代替盐桥,形成新型的电化学振荡,通电逆向电解。文章对这3种类型可逆型化学振荡器的来源、组成、实验方法、理论分析方法和应用前景进行了深入的讨论和分析。它们还可以进一步组合,形成更多的类型。其主要的研究方法为用正交实验确定最佳配方,由相关分析确定影响因素。这些可逆型化学振荡器的关键组分都是表面活性剂,是表面活性剂在非线性化学动力学研究中的又一应用。对可逆型化学振荡器的研究必将大大促进远离平衡态的复杂反应动力学研究及新型交流蓄电池的开发设计。 相似文献
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The role of ion-ion repulsion and ion-phonon coupling in superionic conduction is explored. It is argued that the order-disorder phase transition is not associated with the conductivity discontinuity, but with a higher temperature second order phase transition which has been seen in some superionic conductors and which we predict for others. The specific heat, ion distribution, and conductivity are calculated. 相似文献
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Yurtseven H Tümkaya MH 《Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy》2005,62(4-5):926-930
This study gives our analysis for the frequency shifts of the v2 (1708 cm-1) Raman mode in NH4Cl close to its tricritical (P=1.6 kbar) and second order (P=2.8 kbar) phase transitions. From our analysis, we extract the values of the critical exponent which describes the critical behavior of the Raman frequency shifts for this internal mode for the pressure conditions studied in NH4Cl. Our exponent value of alpha approximately 0.2 for the tricritical phase transition is close to the values of 1/16 (TTc) for the specific heat, predicted from a 3D Ising model. Our exponent values for the second order phase transition (P=2.8 kbar) for TTc are comparable with those reported in earlier studies. 相似文献
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用MonteCarlo法研究了脱附和E-R机理对不可逆催化氧化反应A+1/2B2→AB的相交和自振荡的影响、结果表明,(1)催化剂表面A的脱附使ZGB模型中的一级相变点消失,但对二级相交点的影响很小;当有E-R机理参与时,二级相交点消失,且E-R过程的几率对一级相交点的影响较大;当A的脱附和E-R过程同时起作用时,上述反应不存在相交(2)在ZGB模型中的二级相变点附近,反应速率随时间的变化具有明显的振荡现象,在其它位置主要表现为噪音。引入A的脱附后在二级相交点附近明显的自振荡现象依然存在;当E-R过程起作用时,随着二级相交点的消失,明显的振荡现象亦随之消失.(3)A的脱附和E-R过程对上述反应相变的影响与A的表面扩散对相变的影响有着本质的区别,因为后者只能改变相变点的位置而不能改变相变点的存在状况. 相似文献
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在R-G理论的基础上,考虑了无辐射弛豫过程对位相信号的影响,推导出新的位相公式。研究结果表明,位相(ψ)与吸收系数(β)、热扩散长(μs)、弛豫时的延迟时间(τ)和快弛豫与慢弛豫时的热量比(R)有关。并用此公式解释了稀土化合物PrF3和叶绿素b的位相谱。PrF3的位相由f-f跃迁的吸收系数决定,而叶绿素b两个吸收带之间的位相差是由快弛豫与慢弛豫时放出的热量比不同产生的。 相似文献
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Ruiz-Larrea I. Fraile-Rodríguez A. Arnáiz A. López-Echarri A. 《Journal of Thermal Analysis and Calorimetry》2000,61(2):503-512
New measurements of the (N(CH3 )4 )2 MnBr4 specific heat by adiabatic calorimetry around the ferro- paraelastic phase transition shown by the crystal around 276 K are
compared with previous calorimetric studies on similar tetramethylammonium bromide compounds. The thermodynamic behaviour
of the tribromides and tetrabromides derivatives together with the influence on the phase transition parameters of the cation
and halogen molecular substitutions are examined. The thermal relaxation experiments permit to study the behaviour of the
crystals thermal conduction as a function of the temperature. Finally, the Landau theory for second order phase transitions
is used to describe the thermodynamic behaviour of some of these crystals.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
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Abstract We have extended an earlier molecular model which was developed to explain the double reentrant sequence in highly polar compounds to predict the possibility of a nematic (N1)–nematic (N2) phase transition in such compounds. At moderate densities the dipolar interactions would give rise to an antiparallel near neighbour arrangement of the polar molecules while at higher densities, the dipole–induced dipole and chain–chain dispersion interactions give rise to a parallel configuration. The N1–N2 transition corresponds to a jump in the relative concentration of the two species. Using the mean field approximation we have calculated the phase diagram. The weak first order transition disappears above a critical point as a function of an appropriate parameter. We have also calculated the specific heat anomaly around the transition region. 相似文献
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In the self-consistent field approximation a theory of the orientational ordering is developed for the melt of dimers containing two mesogenic fragments capable of conformational reorganization. Discrete conformations of the dimers (rotational isomers) are characterized by the values of valence angles. Both three-dimensional and two-dimensional melts are considered. It is shown that the presence of bent isomers in the melt reduces the temperature of the phase transition to the anisotropic phase. For a three-dimensional system with first-order phase transition to the ordered state, the jump of the order parameter at the transition point for dimers with conformational flexibility appears to be greater than for rigid dimers. For dimers with two isomers (a linear and a bent one) the order parameter and the statistical weight of the linear isomer at the transition point depend nonmonotonously on the statistical weight of this isomer in the isotropic phase. The effect of “supercooling” of the isotropic phase for linear conformations of flexible dimers is discussed. 相似文献
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Many authors have suggested new forms to describe the surface anchoring energy of the liquid crystal-wall interface, replacing the Rapini-Papoular (RP) formula g s = (1/2) A sin2 theta. If the RP function is considered as the primary approximation, and a lowest order modification is included, then the surface anchoring energy can be represented by g s = (1/2) A sin2 theta(1 + zeta sin2 theta). zeta characterizes the modification to the RP formula and varies for the different energy forms. It is well known that the RP formula predicts a second order Freedericksz transition. This paper points out that the transition can be first order if the modification is taken into account, in which case at the threshold point the tilt angle of the director at the middle layer of the cell, thetam, is finite. The conditions for the existence of the first order transition are obtained; zeta < 0 is required for a first order transition. The approximate expression of the threshold field is also given. 相似文献
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A phenomenological theory is presented to describe the nematic to lamellar phase transition in lyotropic liquid crystals. The problem of the first or second order transition is explored by means of the variation of the surfactant concentration. The possibility of the tricritical point at the nematic to lamellar phase transition is discussed in a phenomenological way. The influence of the electrolyte on this transition is also discussed by varying the coupling between the electrolyte concentration variables and the order parameters. The theoretical prediction is found to be in good qualitative agreement with experimental results. 相似文献